With his last move, White withdrew his knight from d4, counting on driving the enemy rook back into its own camp. But he missed a blow, which became possible as a result of the lack of coordination between the white pieces:

There is also not much chance of holding after 22.gxf3 Qg5+ 23.Ng3 Qxd2–+.

The idea behind Black’s last move is revealed by the simple variation 24.Rc2 Qe3+ 25.Kh1 Rxh2+ 26.Kxh2 Ng4+ 27.Kh1 Qh6+ 28.Kg1 Qh2#.

White resigned.

White threatens an unpleasant check on the kingside, but after...

the position immediately became clear.

White has a couple of checks, but his position is absolutely lost.

White shuts off the long diagonal of the black bishop. This proves sufficient for the black king to feel completely undefended.

Other moves do not save him either:
A) 20...dxe5 21.Ngf6+ Kh8 (21...exf6 22.Qxf6 e4 23.c3+–) 22.Qh4 Bxa1 23.Bg7++–;
B) 20...N4xe5 21.Ndf6+ exf6 22.Nxf6+ Kh8 23.Nxe8 Nd3 24.Qxf7+–;
C) 20...N6xe5 21.Nxe7+ Kh8 22.Qf6#.