12. … QR-B1 13. B-Q3 P-KR3
The threat was: BxKt and BxPch. Had White played P-KKt4 and P-KR4 instead of effecting exchanges in the centre, Black would not have been able to afford this weakening move. But now Black wins the game on the other wing, before White is able to make use of the weakness thus created.
14. BxKt BxB 15. B-B5 KR-Q1 16. BxB PxB 17. Q-Kt6
The Queen must leave the QB file without delay, as Kt-K5 is threatened. Black’s game is already superior; with the exception of the Queen, White has no piece available for the attack on the opposing King.
17. … R-Q3 18. R-QB1 R-R3
Now White must again provide against Black’s Kt-K5, as White’s QKt is needed for the defence of QR2.
19. P-B3 R-Q1
Black intends to move his B and then to advance his KP with an attack on the Queen. The object of the text move is to prevent White from saving himself by an attack on the Rook (Q-B5).
20. R-B2 BxKt
By this exchange Black achieves his object of driving off the Knight by P-Q5, but White has time to give his RP further protection by P-QKt3, This, Black would have prevented by playing B-Kt4 instead of the text move, e.g. 21. P-B4, P-K4; 22. Q-B5, PxKt; 23. PxP, B-B3; 24. PxKt, P-Q5, etc.