It is instructive to watch how this very slight weakness created by Black’s advance of his pawns brings him into trouble. A White Knight settles down at his Q6, which is no longer guarded by the Black QBP, and paralyses the whole of Black’s game. Another factor in White’s superiority of position is the possession of the King’s file. The Black Rook cannot move until the King gets a loophole by a pawn move. As we have seen, such a pawn move often affords an entry to the opposing pieces.
20. P-KB4 B-B1
Not BxP, of course, because of P-QKt3 and Q-Kt2. The Bishop which cannot remain at K3 is to go to Kt2, so that the threat of mate after Q-QB3 may also hold up a White piece.
21. P-B5 B-Kt2 22. Q-K7 Q-QB3 23. R-K2 P-B3
Compulsory, as otherwise P-B6 forces the KtP to advance, which is fatal in any case. After P-Kt3, White would cover his BP and play his Q to KR6. On the other hand, after PxP there is Kt-R5-B6, and Black is in a mating net.
24. Kt-K4 Q-Q4 25. Kt-Q6 B-B3
The threat was QxRch and R-K8 mate.
26. P-KR3
in order to retreat to R2 in case of Q-Q8ch. In a way P-KR3 creates a certain weakness, as the square at Kt3 is now defenceless, but Black has no pieces with which to take advantage of it: his Rook cannot move, his Bishop is on the White squares. If Black had a KB instead, the move would be very doubtful, because then Black might break in through White’s KKt3.