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.