In the game, White offered the more valuable attacking knight for the pawn on c6 by playing 21.Nd5, but after 21...cxd5! 22.Nc7+ Kf7 23.Bxd5+ Ke7, there are no deadly bishop retreats, and continuing the variation with 24.Ra3 (24.Ne6 Qg4+ 25.Kf1 Be4 loses at once) 24...Ne5 25.Ne6 Qxc1+! 26.Qxc1 Rxd5 reveals that the advantage is now with Black.

The threat of a check on c7, plus the powerful major pieces, are deadly, e.g.:

Nothing comes from the check 22...Qg5+ 23.Kf1, and White repulses all threats.

Black is mated, without ever completing the development of his kingside.

The move ...g7-g5 has seriously weakened the long diagonal, which White exploits with the aid of a small tactical operation:

When Black is forced to place a rook in such a position to avoid mate, the correctness of the sacrifice becomes obvious.

Because there is no forced win, White simply strengthens the centralisation of his pieces, so as to be prepared to create threats on the kingside, and also support the advance of his passed pawns.

With piece support, the passed pawns decide the game.

The Bf3 is the most important piece, and, given the weakening of the central light squares, is fully worth a rook.

Of course, Black could get excellent counterplay with 12...Bb7 or 12...Nc6, but the move played in the game is stronger.