user13184: I had the following position with black in a game I recently played:
[FEN "r1bqkbnr/pp1n1ppp/2p1p3/2Pp4/3P4/4PN2/PP3PPP/RNBQKB1R b KQkq - 9 5"]

and proceeded with 5... e5
However, after the game was over, while reviewing it with a chess engine, Stockfish suggested 5... b6 instead and gives -0.16 (equal)
I find this very confusing. I was taught in Nimzowitsch's My System (among in other sources) that one must undermine a pawn chain at it's root, hence e5 seems appropriate.
So my question is: without looking deep into variations, why is in this case b6 a good move (in principle)? Wouldn't white follow up with b4 and have a nice position?

user4223: The other answers so far are all great but I would like to reinforce 2 things.
Firstly, small differences in computer evaluations do not mean much. A computer evaluation is a complicated and imperfect thing.   In fact, Stockfish on my PC rates 1.Nf3 as +0.16, and 1.e4 and 1.d4 both as +0.15.  Does that mean 1.Nf3 is the best opening move?  
Even large differences don't necessarily mean much; for example in a main-line King's Indian (with the closed centre), most computers give White a notable advantage (sometimes even more than +1.0) due to the space advantage, whereas in fact Black's attack will force concessions later that the computer can't see yet.
The best computer evaluation is NOT the answer to "What is the best move in this position?", in general.

Secondly, you talk about which point in the pawn chain you will attack it. Those are general principles that you use in guiding yourself to find ideas for formulating a plan.
However , a plan is NOT a substitute for calculation. Regardless of your plan, you must calculate each move and evaluate the result of that move, and then compare evaluations between those final results.
If you just move based on principles you will lose due to overlooking simple tactics.

user825: We have some excellent answers, in particular the one by @Glorfindel. You should understand:

The evaluation by ...e5 and ...c5 are very close. Both are practically playable. Stockfish has to choose one of those as it's best move but that doesn't mean ...e5 is bad.
5...b6 attacking the c5 pawn is very typical and very common in Queen's pawn opening. We shouldn't even need an engine for this position.
5...b6 is definitely little stronger than ...e5. White is not obliged to play 6.dxe5 after 5...e5, but the c-pawn is harder to defend. 5...b6 gives a small challenge for White to go wrong (the line given by @Glorfindel).


user10233: ...b6 is not an undermining move - it is a direct assault with tempo to win a pawn on c5. There is a difference. One is a tactical motif (...b6), the other is a strategic idea (attacking the base of the pawn chain).
To know the difference and which to apply and when is the beauty of Chess.

user7297: Yes, Black want to play ...e5 eventually (even though the root of the pawn chain is actually at f2). But that doesn't mean you need to play it now, if there are better moves at hand.
It seems Stockfish thinks it's good for Black to liquidate the pawn chain as soon as possible. 6. b4 doesn't bring White much; after 6... a5, White can't protect the b4 pawn with a3 so he needs to push it with 7. b5. After that, many lines are possible but the most likely outcome is that all b- and c-pawns are exchanged and Black retains a very small advantage in development.

user13012: You would like an answer that does not "go into deep variations", but if all such questions could be answered just on general principles, chess would not be as interesting as it is. In this particular case 5.c5 is definitely premature and BOTH methods of undermining the center work well, transferring the initiative (but no more) to Black. The tactics are important. AS Glorfindel observes, after 6..a5 the chain cannot be preserved. If it could, the assessment would change. Usually, though, you either need to strike soon, or you must be able to see that a delayed strike cannot be prevented.
But computer assessments at early stages of the game do not mean much. Stockfish is just saying that both approaches are playable, and you cannot trust it more than that.

13184: <p>I had the following position with black in a game I recently played:</p>

<pre><code>[FEN "r1bqkbnr/pp1n1ppp/2p1p3/2Pp4/3P4/4PN2/PP3PPP/RNBQKB1R b KQkq - 9 5"]
</code></pre>

<p>and proceeded with 5... e5</p>

<p>However, after the game was over, while reviewing it with a chess engine, Stockfish suggested 5... b6 instead and gives -0.16 (equal)</p>

<p>I find this very confusing. I was taught in Nimzowitsch's My System (among in other sources) that one must undermine a pawn chain at it's root, hence e5 seems appropriate.</p>

<p>So my question is: without looking deep into variations, why is in this case b6 a good move (in principle)? Wouldn't white follow up with b4 and have a nice position?</p>
 4223: <p>The other answers so far are all great but I would like to reinforce 2 things.</p>

<p>Firstly, small differences in computer evaluations do not mean much. A computer evaluation is a complicated and imperfect thing.   In fact, Stockfish on my PC rates 1.Nf3 as +0.16, and 1.e4 and 1.d4 both as +0.15.  Does that mean 1.Nf3 is the best opening move?  </p>

<p>Even large differences don't necessarily mean much; for example in a main-line King's Indian (with the closed centre), most computers give White a notable advantage (sometimes even more than +1.0) due to the space advantage, whereas in fact Black's attack will force concessions later that the computer can't see yet.</p>

<p>The best computer evaluation is NOT the answer to "What is the best move in this position?", in general.</p>

<hr>

<p>Secondly, you talk about which point in the pawn chain you will attack it. Those are general principles that you use in guiding yourself to find ideas for formulating a plan.</p>

<p>However , a plan is NOT a substitute for calculation. Regardless of your plan, you must calculate each move and evaluate the result of that move, and then compare evaluations between those final results.</p>

<p>If you just move based on principles you will lose due to overlooking simple tactics.</p>
 825: <p>We have some excellent answers, in particular the one by @Glorfindel. You should understand:</p>

<ul>
<li>The evaluation by ...e5 and ...c5 are very close. Both are practically playable. Stockfish has to choose one of those as it's best move but that doesn't mean ...e5 is bad.</li>
<li>5...b6 attacking the c5 pawn is very typical and very common in Queen's pawn opening. We shouldn't even need an engine for this position.</li>
<li>5...b6 is definitely little stronger than ...e5. White is not obliged to play 6.dxe5 after 5...e5, but the c-pawn is harder to defend. 5...b6 gives a small challenge for White to go wrong (the line given by @Glorfindel).</li>
</ul>
 10233: <p>...b6 is not an undermining move - it is a direct assault with tempo to win a pawn on c5. There is a difference. One is a tactical motif (...b6), the other is a strategic idea (attacking the base of the pawn chain).</p>

<p>To know the difference and which to apply and when is the beauty of Chess.</p>
 7297: <p>Yes, Black want to play ...e5 <em>eventually</em> (even though the root of the pawn chain is actually at f2). But that doesn't mean you need to play it now, if there are better moves at hand.</p>

<p>It seems Stockfish thinks it's good for Black to liquidate the pawn chain as soon as possible. 6. b4 doesn't bring White much; after 6... a5, White can't protect the b4 pawn with a3 so he needs to push it with 7. b5. After that, many lines are possible but the most likely outcome is that all b- and c-pawns are exchanged and Black retains a very small advantage in development.</p>
 13012: <p>You would like an answer that does not "go into deep variations", but if all such questions could be answered just on general principles, chess would not be as interesting as it is. In this particular case 5.c5 is definitely premature and BOTH methods of undermining the center work well, transferring the initiative (but no more) to Black. The tactics are important. AS Glorfindel observes, after 6..a5 the chain cannot be preserved. If it could, the assessment would change. Usually, though, you either need to strike soon, or you must be able to see that a delayed strike cannot be prevented.</p>

<p>But computer assessments at early stages of the game do not mean much. Stockfish is just saying that both approaches are playable, and you cannot trust it more than that.</p>
