|Once: This is the position after 16...Bxd7
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Black is playing the standard h file hack - the club player's favourite. Wait until your opponent has castled and then throw everything at his castled king, with rook(s) and queen ganging up on the h file. It's the Barney Rubble of the chess world - crude but surprisingly effective.
The h file hack is particularly nasty against a fianchetto (or any other dish made out of pasta). It's the one Fischer quote that nearly everybody knows "I'd won dozens of skittles games in analogous positions and had it down to a science - pry open the KR-file, sac, sac ... mate!"
This position is where white has to choose how he is going to respond. Just as black has a standard attack going, white has a number of standard responses. He can (1) try to shore up his defences (2) run his king to the queenside (3) counter-attack in the centre or on the queen's wing.
That probably explains why white plays 17. b4 at this point. It's a mixture of queenside attack and a sort of defence (if followed up by Rb1-Rb3).
White presumably reckons that black is going to castle queenside sometime soon, so let's prepare a counter attack for when he gets there.
But this is the point when discretion is probably the better part of valour. Or "valor" for those who don't have a passport. Surely this was the time to play 17. Bf2 (defending g3) or 17. Kf2 (running like a scaredy cat)?
"For he who fights and runs away;
May live to fight another day;
But he who is in battle slain.
Can never rise and fight again."
But he almost certainly won't get the girl in the fifth reel.
Fritzie reckons that 17. Kf2 and 17. Bf2 keep the black advantage down to less than a prawn. And anything less than a prawn is usually nouvelle cuisine - mightily expensive but ultimately not satisfying.
But 17. b4? Nope, sorry. Fritzie winks his disapproving red eye (which BTW always reminds me of a Terminator) and the eval drops to an skynettish end of the world with -4.5. That's not prawn in a puddle stylish, it's a full half pounder with cheese, bacon, onion rings and fries (which may or may not include real potatoes) and the girl is smiling at you and saying "do yous wanna go large widdat?"