Very clever. It is worth a pawn to swap off Black's active pieces, so that he is left with only bad ones.
13...fxg4 14.Nxg4 h5 15.Ne5+ Ke8 16.Kh2 g5 17.Kg3 gxf4+ 18.Kxf4 Rf6+ 19.Kxe4 Rxf2 20.Rg1 Bxd6 21.Nxd6+ Kf8 22.Raf1
Before getting on with the kingside attack, Black prevents the opponent's idea of 2 Nc4.