19...Nxf2

Black continues with the real sacrifice, a Knight. This is the only move that justifies 18...Rxf2. Although White now accepts the Knight sacrifice, there was an alternative. Korchnoi noted

Better chances were offered by 20.Bxg6. If 20...fxg6 then 21.Kxf2 and Black's heavy pieces have no squares on which to intrude. Therefore Black would have to make the sacrifice 20...Nxh3+ 21.gxh3 hxg6 with excellent attacking chances.

In Korchnoi's last line, the straightforward 22.Nd2 fails to 22...Qg5+ 23.Kh1 Re2. A better continuation would be 22.Qd2 Qh4 23.Na3 Re4, but even here White can't cope with the Queen and Rook bearing down on the exposed King.

Next move: 20.Kxf2