If Black recaptures with the pawn, he must lose a move with the Bishop in order to avoid getting an isolated doubled pawn after 6. Q-B3. The doubled pawn which Black may get after the move in the text would not be isolated, and therefore not necessarily weak. It could become a weakness if Black were to castle on the King’s side. But otherwise it might even become a source of strength, supporting, as it would, an advance of Black’s KP against the White centre.
6. Kt-B3 P-B4?
Black should retain the option of castling QR, in case White exchanges at his KB6; P-QKt3 and B-Kt2 would have been better.
7. BxKt PxB
8. Q-Q2 BxKt
9. QxB Kt-Q2
10. R-Q1 R-KKt1
11. PxP QxP
12. Q-Q2 Q-Kt3
guarding against the mate at Q1 before moving the Kt. But this would have been better effected by Q-K2. After Q-Kt3 the Knight cannot move yet because of B-Kt5ch.
Diag. 137
13. P-B3 P-QR3 14. Q-B2 P-B4 15. P-KKt3 Kt-B4 16. B-Kt2 Q-B2
Black wishes to push on the KP. White, however, prevents this at once.
17. Q-K2 P-Kt4 18. Castles B-Kt2