user1485: I hope it's OK with a question on the fringes of existing opening theory.
If white wants to combine pawn moves f4 and b4 early in the opening, he has to be aware of  the possibility of black snatching a pawn. One example is Larsen-Raizman from the 1958 Olympiad 1.f4 Nf6 2.Nf3 g6 3.b4, where black could have gone 3...Nd5!? with complicated play.
Even earlier possible pawn losses are 1.f4 d5 2.b4 Qd6 and 1.f4 Nf6 2.b4 Nd5. I want to focus on this last variation.
    [FEN ""]
1.f4 Nf6 2.b4 Nd5

Before I thought this was good for black, but now I'm not so sure anymore.
3.e4 Nxf4 4.d4 looks like great compensation, although ironically I would prefer the b4 pawn back on b2. Black is better advised to play 3...Nxb4, for instance 4.d4 d5. A possible improvement for white is 3.d4 and after 3...Nxb4 start chasing the knight around.
Questions:
Is 2...Nd5 advisable for black?
How should white react?

user17: F4 is a reasonable opening move, to discourage Black's e5. He played Nf6 in a similar vein to discourage your e4.
But the followup move, b4, was bad. Not only does it allow the knight fork, but it has no relation to f4. (The fork is really the symptom, and not the disease). Most logically, you might follow with another move on the kingside such as Nf3 or g3. If you want to play on the queen side, the logical moves are Nc3, or d3, both of which would protect a future pawn on e4 from the knight on f6.

user1506: I think this depends on how you play chess as a human and how good the players are. Some people want to take every pawn they can and then they are defending their king, swapping pieces and then they win the game in the end game. Some want to give them away for some initiative (like white in our example). I think an engine will definitely play Nd5 and grab the pawn(s).
I would play Nd5 and take b4 instead of f4, because white gets aggressive play after e4 Nxf4 d4, especially the half open f file is very nice.
If white plays c3 after Nd5, I would take f4.

user1493: This question led me to spend two hours of my life looking at a line that I most likely never will get to play.  I went into it being quite confident that Nd5 was a bad move simply because White would get too much development for the cost of a simple pawn.  Analysing back and forth with Houdini has nevertheless convinced me that Nd5 is perfectly playable.
The similarities to the Sokolsky Opening
To get a feel for the b4 pawn it helps to understand a variation from the Sokolsky Opening.  It goes like this:
[FEN ""]

1. b4 e6  2. Bb2 Nf6  3.b5

Here the pawn on b5 is slightly annoying.  It cannot be easily removed as a6 can be answered by a4 (it does not matter if the rooks are swapped off as White intends to castle short anyway) and c6 can be answered by e3 when Black must take away from the centre to remove the pawn.
In the game, however, Black can simply play a6 in the similar position as White can't answer with a4:
[FEN ""]

1.f4 Nf6  2. b4 e6  3.b5 a6

So after 2...e6 White is only left with 3 a3, which, although reasonable, is another pawn move on the wing.  Black will be very solid after playing simple development like d5, Bd6 and 0-0.  This is slightly better than one of the main lines of the Sokolsky Opening, which is not that good to start with (but where White often ends up playing f4 later).
Houdini 3 on 2...Nd5
The currently strongest engine on the planet is Houdini 3.  When you let it analyse the position before 2...Nd5 to a depth of 25 it likes 2...d6 (claiming a small advantage for Black) the best (with 2...d5 as a strong second).  2...Nd5 does not come up as a top five alternative.  Houdini is initially not impressed by grabbing the pawn.
White plays 3 e4
Nevertheless, 2...Nd5 is still an interesting alternative and certainly gives more entertaining variations to look at.  First let us take a look at what happens if White plays 3 e4 after 2...Nd5 and then Black takes the pawn at b4:
[FEN ""]

1.f4 Nf6  2.b4 Nd5  3.e4 Nxb4  4.c3 N4c6  5.d4 d5  6.Nd2 dxe4  7.Nxe4

To explain this variation it is important to know that Black's hope and main idea against a setup with f4-e4-d4 is to play d5 and develop Bc8 to f5or g4.  In the variation above it turns out Black can't play either.  Bg4 simply hangs the bishop and Bf5 runs into Ng3 chasing it back.  Here I (and Houdini) believe White has compensation for the pawn (although perhaps nothing more).
So, let us look at the alternative.  Initially I was sure that 3...Nxf4 must be horrible for Black, but it turns out it is good after 4. d4 e5!:
[FEN ""]

1.f4 Nf6 2.b4 Nd5 3.e4 Nxf4 4.d4 e5 5.dxe5 Ne6

As seen above, if White makes the natural capture on e5, Black just retreats the knight.  Material is now even, but Black has two threats; The killing Qh4+ and just grabbing the b4 pawn.  So Black will end up a pawn up with a much better pawn structure and no imminent attack from White.
Instead White must play:
[FEN ""]

1.f4 Nf6 2.b4 Nd5 3.e4 Nxf4 4.d4 e5 5.Nf3 Ne6 6.d5 Nd4 7.c3 (7.Ng5) Nxf3+ 8.Qxf3 a5

In the above variation please note that 6 Nxe5 runs into 6...Qxh4+.  The same idea works against 7 Nxd4.  And despite all the knight moves Black is left clearly better in this variation too.  If you want to play this variation with White you need to try the chaotic 7.Ng5!?, hoping to trap the knight at d4.  Have fun analysing that.
White plays 3 d4
I believe 3 d4 is a strong alternative for White.  If Black takes the pawn on b4 it transposes to a variation we have already looked at:
[FEN ""]

1.f4 Nf6 2.b4 Nd5 3.d4 Nxb4 4.c3 N4c6 5.e4 d5 6.Nd2 dxe4 7.Nxe4

So, as I mentioned in a comment to the question, perhaps Black can try the amazing 3...Nf6 with the simple plan of d5 and Bf5.  In this variation White will have more space, but also more weaknesses (and, no, Black is not behind in development):
[FEN ""]

1.f4 Nf6 2.b4 Nd5 3.d4 Nf6

White plays 3 Bb2
Seeing that White will lose one pawn anyway, why not play a development move like  and see which pawn Black decides to take?  It seems that Black can just grab the f4 pawn and get away with it.  Let us see what happens if White tries to push Black off the board:
[FEN ""]

1.f4 Nf6 2.b4 Nd5 3.Bb2 Nxf4 4.e3 Ng6 5.h4 h5 6.Be2 d5 7.Bxh5 Qd6

White won the h5 pawn, but now the threat of Qg3+ is just killing White.  Instead, if White tried some quiet development moves:
[FEN ""]

1.f4 Nf6 2.b4 Nd5 3.Bb2 Nxf4 4.e3 Ng6 5.Nf3 e5 6.Nxe5 Nxe5 7.Bxe5 Nc6

Black gets a small advantage by the precise 5...e5 which trades the not so optimally placed Ng6 (Black is still a pawn up).  Note that the alternative 3 Nf3 could easily transpose as it does not seem that White can improve much
Conclusion
The fork 2...Nd5 works out well for Black, despite Houdini's initial reluctance towards it.  It seems White should play 3 d4 to equalise.  I would certainly try 2...Nd5 should 1 f4 Nf6  2 b4 be played against me.

1485: <p>I hope it's OK with a question on the fringes of existing opening theory.</p>

<p>If white wants to combine pawn moves <code>f4</code> and <code>b4</code> early in the opening, he has to be aware of  the possibility of black snatching a pawn. One example is Larsen-Raizman from the 1958 Olympiad <code>1.f4 Nf6 2.Nf3 g6 3.b4</code>, where black could have gone <code>3...Nd5!?</code> with complicated play.</p>

<p>Even earlier possible pawn losses are <code>1.f4 d5 2.b4 Qd6</code> and <code>1.f4 Nf6 2.b4 Nd5</code>. I want to focus on this last variation.</p>

<pre><code>    [FEN ""]
1.f4 Nf6 2.b4 Nd5
</code></pre>

<p>Before I thought this was good for black, but now I'm not so sure anymore.
<code>3.e4 Nxf4 4.d4</code> looks like great compensation, although ironically I would prefer the b4 pawn back on b2. Black is better advised to play <code>3...Nxb4</code>, for instance <code>4.d4 d5</code>. A possible improvement for white is <code>3.d4</code> and after <code>3...Nxb4</code> start chasing the knight around.</p>

<p>Questions:</p>

<p>Is <code>2...Nd5</code> advisable for black?</p>

<p>How should white react?</p>
 17: <p>F4 is a reasonable opening move, to discourage Black's e5. He played Nf6 in a similar vein to discourage your e4.</p>

<p>But the followup move, b4, was bad. Not only does it allow the knight fork, but it has no relation to f4. (The fork is really the symptom, and not the disease). Most logically, you might follow with another move on the kingside such as Nf3 or g3. If you want to play on the queen side, the logical moves are Nc3, or d3, both of which would protect a future pawn on e4 from the knight on f6.</p>
 1506: <p>I think this depends on how you play chess as a human and how good the players are. Some people want to take every pawn they can and then they are defending their king, swapping pieces and then they win the game in the end game. Some want to give them away for some initiative (like white in our example). I think an engine will definitely play <code>Nd5</code> and grab the pawn(s).</p>

<p>I would play <code>Nd5</code> and take <code>b4</code> instead of <code>f4</code>, because white gets aggressive play after <code>e4 Nxf4 d4</code>, especially the half open <code>f</code> file is very nice.</p>

<p>If white plays <code>c3</code> after <code>Nd5</code>, I would take <code>f4</code>.</p>
 1493: <p>This question led me to spend two hours of my life looking at a line that I most likely never will get to play.  I went into it being quite confident that <code>Nd5</code> was a bad move simply because White would get too much development for the cost of a simple pawn.  Analysing back and forth with Houdini has nevertheless convinced me that <code>Nd5</code> is perfectly playable.</p>

<p><strong>The similarities to the Sokolsky Opening</strong></p>

<p>To get a feel for the <code>b4</code> pawn it helps to understand a variation from <a href="http://en.wikipedia.org/wiki/Sokolsky_Opening" rel="noreferrer">the Sokolsky Opening</a>.  It goes like this:</p>

<pre><code>[FEN ""]

1. b4 e6  2. Bb2 Nf6  3.b5
</code></pre>

<p>Here the pawn on <code>b5</code> is slightly annoying.  It cannot be easily removed as <code>a6</code> can be answered by <code>a4</code> (it does not matter if the rooks are swapped off as White intends to castle short anyway) and <code>c6</code> can be answered by <code>e3</code> when Black must take away from the centre to remove the pawn.</p>

<p>In the game, however, Black can simply play <code>a6</code> in the similar position as White can't answer with <code>a4</code>:</p>

<pre><code>[FEN ""]

1.f4 Nf6  2. b4 e6  3.b5 a6
</code></pre>

<p>So after <code>2...e6</code> White is only left with <code>3 a3</code>, which, although reasonable, is another pawn move on the wing.  Black will be very solid after playing simple development like <code>d5</code>, <code>Bd6</code> and <code>0-0</code>.  This is slightly better than one of the main lines of the Sokolsky Opening, which is not that good to start with (but where White often ends up playing <code>f4</code> later).</p>

<p><strong>Houdini 3 on <code>2...Nd5</code></strong></p>

<p>The currently strongest engine on the planet is <a href="http://www.cruxis.com/chess/houdini.htm" rel="noreferrer">Houdini 3</a>.  When you let it analyse the position before <code>2...Nd5</code> to a depth of 25 it likes <code>2...d6</code> (claiming a small advantage for Black) the best (with <code>2...d5</code> as a strong second).  <code>2...Nd5</code> does not come up as a top five alternative.  Houdini is initially not impressed by grabbing the pawn.</p>

<p><strong>White plays <code>3 e4</code></strong></p>

<p>Nevertheless, <code>2...Nd5</code> is still an interesting alternative and certainly gives more entertaining variations to look at.  First let us take a look at what happens if White plays <code>3 e4</code> after <code>2...Nd5</code> and then Black takes the pawn at <code>b4</code>:</p>

<pre><code>[FEN ""]

1.f4 Nf6  2.b4 Nd5  3.e4 Nxb4  4.c3 N4c6  5.d4 d5  6.Nd2 dxe4  7.Nxe4
</code></pre>

<p>To explain this variation it is important to know that Black's hope and main idea against a setup with f4-e4-d4 is to play <code>d5</code> and develop <code>Bc8</code> to <code>f5</code>or <code>g4</code>.  In the variation above it turns out Black can't play either.  <code>Bg4</code> simply hangs the bishop and <code>Bf5</code> runs into <code>Ng3</code> chasing it back.  Here I (and Houdini) believe White has compensation for the pawn (although perhaps nothing more).</p>

<p>So, let us look at the alternative.  Initially I was sure that <code>3...Nxf4</code> must be horrible for Black, but it turns out it is good after <code>4. d4 e5!</code>:</p>

<pre><code>[FEN ""]

1.f4 Nf6 2.b4 Nd5 3.e4 Nxf4 4.d4 e5 5.dxe5 Ne6
</code></pre>

<p>As seen above, if White makes the natural capture on e5, Black just retreats the knight.  Material is now even, but Black has two threats; The killing <code>Qh4+</code> and just grabbing the <code>b4</code> pawn.  So Black will end up a pawn up with a much better pawn structure and no imminent attack from White.</p>

<p>Instead White must play:</p>

<pre><code>[FEN ""]

1.f4 Nf6 2.b4 Nd5 3.e4 Nxf4 4.d4 e5 5.Nf3 Ne6 6.d5 Nd4 7.c3 (7.Ng5) Nxf3+ 8.Qxf3 a5
</code></pre>

<p>In the above variation please note that <code>6 Nxe5</code> runs into <code>6...Qxh4+</code>.  The same idea works against <code>7 Nxd4</code>.  And despite all the knight moves Black is left clearly better in this variation too.  If you want to play this variation with White you need to try the chaotic <code>7.Ng5!?</code>, hoping to trap the knight at d4.  Have fun analysing that.</p>

<p><strong>White plays <code>3 d4</code></strong></p>

<p>I believe <code>3 d4</code> is a strong alternative for White.  If Black takes the pawn on <code>b4</code> it transposes to a variation we have already looked at:</p>

<pre><code>[FEN ""]

1.f4 Nf6 2.b4 Nd5 3.d4 Nxb4 4.c3 N4c6 5.e4 d5 6.Nd2 dxe4 7.Nxe4
</code></pre>

<p>So, as I mentioned in a comment to the question, perhaps Black can try the amazing <code>3...Nf6</code> with the simple plan of <code>d5</code> and <code>Bf5</code>.  In this variation White will have more space, but also more weaknesses (and, no, Black is not behind in development):</p>

<pre><code>[FEN ""]

1.f4 Nf6 2.b4 Nd5 3.d4 Nf6
</code></pre>

<p><strong>White plays <code>3 Bb2</code></strong></p>

<p>Seeing that White will lose one pawn anyway, why not play a development move like  and see which pawn Black decides to take?  It seems that Black can just grab the <code>f4</code> pawn and get away with it.  Let us see what happens if White tries to push Black off the board:</p>

<pre><code>[FEN ""]

1.f4 Nf6 2.b4 Nd5 3.Bb2 Nxf4 4.e3 Ng6 5.h4 h5 6.Be2 d5 7.Bxh5 Qd6
</code></pre>

<p>White won the <code>h5</code> pawn, but now the threat of <code>Qg3+</code> is just killing White.  Instead, if White tried some quiet development moves:</p>

<pre><code>[FEN ""]

1.f4 Nf6 2.b4 Nd5 3.Bb2 Nxf4 4.e3 Ng6 5.Nf3 e5 6.Nxe5 Nxe5 7.Bxe5 Nc6
</code></pre>

<p>Black gets a small advantage by the precise <code>5...e5</code> which trades the not so optimally placed <code>Ng6</code> (Black is still a pawn up).  Note that the alternative <code>3 Nf3</code> could easily transpose as it does not seem that White can improve much</p>

<p><strong>Conclusion</strong></p>

<p>The fork <code>2...Nd5</code> works out well for Black, despite Houdini's initial reluctance towards it.  It seems White should play <code>3 d4</code> to equalise.  I would certainly try <code>2...Nd5</code> should <code>1 f4 Nf6  2 b4</code> be played against me.</p>
