user16311: In this game, http://www.chessgames.com/perl/chessgame?gid=1365631
The engine says that 20. Na5 is a blunder. However, I fail to understand why it is. Also, I am surprised at White's choice of moves here with c4 instead of Qxd6. Essentially he is giving a Bishop for free.
 [title "Bronstein vs Winter"]
 [fen "r2r2k1/pp3p1p/1n1B2p1/3P4/2q5/1N3P2/PbPQ1RPP/3R2K1 w - - 1 20"]

 1. Na5 Qa6 2. Qb4 Rxd6 3. c4?

What is the problem with 22. Qxd6 Qxa5 line where White gets a Rook and a pawn for his Knight and Bishop, the position looks equal. The engine says it is -3 for Black and gives a long long line but I am unable to see how it gives Black an advantage.

user12873: In the starting position, White should immediately protect his Bishop with 1.Qb4. Then, after 1...Qxb4 2.Bxb4 Rxd5, the endgame is close to equal.
Instead, after 1.Na5 Qa6, White's bishop is still under attack and his knight is vulnerable. The game continued 2.Qb4 Rxd6, and now if White plays 3.Qxd6, Black has 3...Qxa5. Black has won two pieces for a Rook since White's queen got overloaded.
You're right though that 3.Qxd6 was a better move than 3.c4, but only by about 0.40 according to Fritz. Both moves are under -2. The point behind 3.c4 is that White's f2-rook is attacking Black's b2-bishop and threatening to take it. The bishop wasn't already threatened by White's queen since after Qxb2 Black has ...Qxa5.

16311: <p>In this game, <a href="http://www.chessgames.com/perl/chessgame?gid=1365631" rel="nofollow noreferrer">http://www.chessgames.com/perl/chessgame?gid=1365631</a></p>

<p>The engine says that 20. Na5 is a blunder. However, I fail to understand why it is. Also, I am surprised at White's choice of moves here with c4 instead of Qxd6. Essentially he is giving a Bishop for free.</p>

<pre><code> [title "Bronstein vs Winter"]
 [fen "r2r2k1/pp3p1p/1n1B2p1/3P4/2q5/1N3P2/PbPQ1RPP/3R2K1 w - - 1 20"]

 1. Na5 Qa6 2. Qb4 Rxd6 3. c4?
</code></pre>

<p>What is the problem with 22. Qxd6 Qxa5 line where White gets a Rook and a pawn for his Knight and Bishop, the position looks equal. The engine says it is -3 for Black and gives a long long line but I am unable to see how it gives Black an advantage.</p>
 12873: <p>In the starting position, White should immediately protect his Bishop with 1.Qb4. Then, after 1...Qxb4 2.Bxb4 Rxd5, the endgame is close to equal.</p>

<p>Instead, after 1.Na5 Qa6, White's bishop is still under attack and his knight is vulnerable. The game continued 2.Qb4 Rxd6, and now if White plays 3.Qxd6, Black has 3...Qxa5. Black has won two pieces for a Rook since White's queen got overloaded.</p>

<p>You're right though that 3.Qxd6 was a better move than 3.c4, but only by about 0.40 according to Fritz. Both moves are under -2. The point behind 3.c4 is that White's f2-rook is attacking Black's b2-bishop and threatening to take it. The bishop wasn't already threatened by White's queen since after Qxb2 Black has ...Qxa5.</p>
