user2042: I was playing a game as Black against computer and came up a tactical move that defends me from pressure. I have expected a normal retreat of the queen yet computer kept sacrificing the knight like it was lost anyway, therefore this question :
Did I miss a win for Black or the position is just fine for White?
Here is the position:
[Title "White to move"]
[fen "4r1k1/3q1ppp/pp6/2QN1n2/1P6/8/P4PPP/3R2K1 w - - 0 1"] 

White kept playing Nf6+ or Qxb6 but I believe that any retreat of the queen is fine, what did I miss?

user70: Black has no win.  White scoops up the b6 pawn and enjoys the advantage.
[Title "White to move"]
[fen "4r1k1/3q1ppp/pp6/2QN1n2/1P6/8/P4PPP/3R2K1 w - - 0 1"] 


     1.Qc1 Qa4 2.a3 Ne7 3.Nxb6 Qb5 4.Nc4 Nf5 5.g3 h6 6.Nd6 Nxd6 7.Rxd6 a5 8.bxa5 Qxa5 9.Qc6 Qa8 10.a4 Kf8 11.Kg2 Qxc6+ 12.Rxc6 Ra8 13.Rc4 Ke7 14.Kf3 Kd6 15.Rd4+ Kc6 16.h4 g6 17.Rf4 Ra7 18.Ke3 Kd5 19.Kd3 Ke5 20.Rb4

So says Stockfish 5.  Everything past move 5 or so is probably suspect but you get the idea.

user2429: My suspicion is that the computer is calculating at a depth of 3-ply. At 3-ply depth, it sees that the following variations are winning - 
1. Nf6+ gxf6 2. Rxd7 and 
1. Qxb6 Qxd5 2. Rxd5
winning material. 
If it was at 4-ply or more, it would have seen 2...Re1# in both lines and rejected those lines. 
To answer your question, no, Black doesn't have a win in this position. White can play 1. Qc1 and white is fine. 

2042: <p>I was playing a game as Black against computer and came up a tactical move that defends me from pressure. I have expected a normal retreat of the queen yet computer kept sacrificing the knight like it was lost anyway, therefore this question :</p>

<p>Did I miss a win for Black or the position is just fine for White?</p>

<p>Here is the position:</p>

<pre><code>[Title "White to move"]
[fen "4r1k1/3q1ppp/pp6/2QN1n2/1P6/8/P4PPP/3R2K1 w - - 0 1"] 
</code></pre>

<p><em>White kept playing <code>Nf6+</code> or <code>Qxb6</code> but I believe that any retreat of the queen is fine, what did I miss?</em></p>
 70: <p>Black has no win.  White scoops up the <code>b6</code> pawn and enjoys the advantage.</p>

<pre><code>[Title "White to move"]
[fen "4r1k1/3q1ppp/pp6/2QN1n2/1P6/8/P4PPP/3R2K1 w - - 0 1"] 


     1.Qc1 Qa4 2.a3 Ne7 3.Nxb6 Qb5 4.Nc4 Nf5 5.g3 h6 6.Nd6 Nxd6 7.Rxd6 a5 8.bxa5 Qxa5 9.Qc6 Qa8 10.a4 Kf8 11.Kg2 Qxc6+ 12.Rxc6 Ra8 13.Rc4 Ke7 14.Kf3 Kd6 15.Rd4+ Kc6 16.h4 g6 17.Rf4 Ra7 18.Ke3 Kd5 19.Kd3 Ke5 20.Rb4
</code></pre>

<p>So says Stockfish 5.  Everything past move 5 or so is probably suspect but you get the idea.</p>
 2429: <p>My suspicion is that the computer is calculating at a depth of 3-ply. At 3-ply depth, it sees that the following variations are winning - </p>

<p><code>1. Nf6+ gxf6 2. Rxd7</code> and </p>

<p><code>1. Qxb6 Qxd5 2. Rxd5</code></p>

<p>winning material. </p>

<p>If it was at 4-ply or more, it would have seen <code>2...Re1#</code> in both lines and rejected those lines. </p>

<p>To answer your question, no, Black doesn't have a win in this position. White can play <code>1. Qc1</code> and white is fine. </p>
