White did not relish the idea of exd4, which would have left him with an isolani. The White pieces are massing in the center; if Black does not defuse the situation quickly, there will be no endgame.

White can escape the pawn fork, but it will cost him dearly.

Black has secured a connected passer on the 6th rank which severely restricts White's mobility. White must now keep this pawn under constant watch with heavy pieces.

The White queen and bishops look menacing, so Black forces an exchange of bishops.

Black has blunted the White K-side attack and at the same time lined up his rooks on the crucial files, preparing to advance the Q-side pawn majority.

White has managed to maneuver his remaining bishop into position to blockade the Black pawns. The bishop will prove to be the key to White's defense.

This gets the Black K off the g-file, thereby freeing the Black Q from having to hold the f6 pawn against the threat of Bxf6. In the final position, it will be seen that this move has a further significance, unforeseen at this juncture.

White's K-side attack has fizzled out, so White now goes into blockading mode to hold off the Black pawn charge.

A blocking sacrifice, which White cannot refuse.

Black has given back the extra pawn and thereby secured a second passer; the White c3 pawn cramps White's bishop and c1 rook.