To prevent Black’s P-B4.
At first sight it seems as if the QBP ought to move to B4, as the advance of the QKtP has weakened it. But White dares not allow a Black Knight to settle at Q5.
12. … P-Q3 13. P-B4!
Diag. 108
Black threatens to play K-R1 in order to play P-B4. White’s position would then be very bad, and therefore he rightly decides to anticipate the move, even at the cost of a pawn. In order to gain the QBP Black must waste a number of moves with the Q, and White gains time for a King’s side attack. The pawn sacrifice is very promising indeed.
13. … Q-B3 14. Castles QxQBP 15. R-B3
There seem to be many threats here, and the position is a difficult one to fathom. After disentangling his Queen, Black tries very hard to force his P-B4. As soon as he succeeds in this he has a won game, for the open file is available both for defence and counter-attack.
15. … Q-Q5 16. K-R1 B-K3 17. R-QB1 BxB 18. RxB Q-Kt7
Q-B3 is impossible apart from the fact that it would block the KBP, e.g. 18. … Q-B3; 19. P-K5, PxP; 20. Kt-K4, etc.