The King's move is necessary, else Black could play 15.... Kt to K 4; 16. Kt takes Kt, B P takes Kt, and White could not reply 17. P to B 4 (an essential move for the attack), because Black would then win a piece with 17.... P to B 5; discovering check, and attacking the Bishop at the same time.
| ... | 15. | B to B 2 |
This Bishop being now comparatively useless at Kt 3, since the White King has quitted the black diagonal, retires, in order to make room for the advance of the pawns.
| 16. | Q R to B sq. | 16. | R to Kt sq. |
To support the advance of the Kt P.
| 17. | Kt to Kt 3 | 17. | P to Kt 4 |
| 18. | Kt to B 5 | 18. | P to B 5 |
It is an essential point in the defense to prevent White's Knight from being posted at K 6. If White is able to accomplish this, his attack becomes so powerful that it cannot be withstood. It would, therefore, be bad for Black to play, instead of the move given in the text, 18.... B takes Kt, because of 19. P takes B, Kt to K 4; 20. B takes Kt, B P takes B; 21. Kt to Kt 5, followed by 22. Kt to K 6 (the dangerous move just pointed out).
| 19. | B to K 2 | ... |
Not 19. B to Kt sq. White wants to keep a pressure upon Black's pawns. The position is now as shown in Fig 9. Neither side has as yet gained any marked advantage.