OrangeCounty wrote on 02/18/11 at 18:52:44:
I am reasoning by analogy here, which is always bad in the Dragon, because everything depends on the tactical details, but since this is the Bg5 line, can't White ignore the pawn advance rather than accepting a pawn sacrifice, decentralizing his knight, and giving Black a tempo or two for the attack?
White hasn't weakened his pawn chain yet (no f3-f4), so perhaps just 14 g4 b4 15 Nd5 (the Rook hasn't reached c5 yet) - White can round up the b4 pawn later, if allowed, but for now the obligation is to get on with the attack, which means removing the f6 knight. White doesn't always try to mate Black very hard (anymore), but that doesn't mean he can't take a shot at it if Black is making pawn moves. I used to try to play b5 in a different position: 11 h4 Ne5 12 g4 b5, but there the idea for White is to ignore the pawn, because he wants to play Nd5 anyway.
Why is this different?
Well, if Black was forced to react to 14.g4 with 14...b4, you would be perfectly right. However Black plays 14...a5! with the idea of harrassing the bishop on b3. For example 15.gxh5 a4 and the position is very concrete. The idea of 13...b5 is not so much to follow up with 14...b4, but to play a5 next without giving White the opportunity to lock up the position with a4.
That said, White may consider to wait for one move (without weakening his f3-pawn) and only take on b5 after Black has played 14...a5. For example 14.Kb1!? is an interesting move. Then 14...b4? runs into 15.Nd5 and 14...a5 can be met by 15.Ndxb5 with idea 16.a4. However Black can play 14...Nc4 15.Bxc4 Rxc4, which is perfectly fine. If Black really wants a sharp game, he can also play 14...a5!? 15.Ndxb5 a4 16.Bxa4 Nc4, which gives Black good compensation for the pawns. Even after the stronger 15.Ncxb5! a4 16.Bxa4 Black obtains reasonable compensation with 16...Qb6 or 16...Rc5.