Restricting White's intended queenside advance,which could have become real after 1...Bxa3?1 2.Nxa3 , followed by Nc2, a2-a3 and b3-b4.
Preparing ...h7-h5 with an attack. Thanks to his powerful knight, Black enjoys good compensation for the pawn.
Unity Chess Club
Eingorn - Agrest Chalkidiki 2002 White to move
1.Qxa6! Nxa6 2.h6!
In order to imprison Black's bishop just before he vacates the f8-square by ...Rfe8 or ...Rfc8.
From now White is basically a piece up and should just try to neutralize Black's temporary initiative.
Swapping the more active (to say the least) of Black's minor pieces. This also helps to improve his own bishop,
12...Nxg5 13.Bxg5 Kf5 14.Be7! Rfe8 15.Rxc8 Rxc8 16.Rf1+ Ke6 17.Bd6 a6 18.g4 g5 19.Rf5 Rg8 20.Ba3!