White defends his q-side indirectly with the counter-threat of 20. Rb1 in case Black plays 19. ... Qxb2. Furthermore, the weakened d6 pawn is now double attacked and only once defended. Both sides are slugging away, trying to wrest the initiative; whoever blinks, loses.

Black elects to let the d-pawn fall without defending it, e.g., by 19. ... Rd8. 19. ... Be5 would have been met by 20. f3-f4.

This penetration by the knight will lastingly hamper Black's defence in the coming k-side attack.

Positional assessment at mid-game: White has let the q-side go in exchange for a promising attack against the Black king position. If White can play h4-h5 and trade off the g6 pawn, or provoke h7-h5 to prevent the advance of the h4 pawn, then a double piece sacrifice (N and B) on g6 may be sufficient to breach the Black king position. Black, on the other hand, has stong endgame prospects due to his passed a-pawn, supported by the QR and the bishop pair. If White's k-side attack fails, the endgame will favour Black.

White had threatened to put the KR too on the 7th rank; Black must prevent this.

Of course not ... RxRb8?? 27. QxRb8+ Bf8; 28. QxBf8 mate.

White threatens 29. h5 breaking open the Black king position. If Black allows 30. h5xg6, then hxg6; 31. Bxg6! and Black is lost. (The tempting-looking 29. ... Be5 pinning the Q fails to 30. RxR+, Kg7; 31. Rg8 mate.)

Both sides are slugging away, attacking and defending at once. The text move pins the White bishop against the queen, thus preventing the immediate threat of Bxg6, while at the same time attacking White's f2 pawn.

The attack on the White bishop is only apparent; 30. ... f5xBe4 would be met by 31. RxRf8+, BxRf8; 32. QxBf8+, Bg8; QxBg8 mate.

White continues to hammer away at Black's king.