White’s knight is on a light square, as are Black’s king, queen, and rook. You try first to fork the two more valuable pieces and so see that Nxf6+ attacks the king and queen. The trouble is that g7xN follows. Black’s g-pawn can’t be pinned, captured, or distracted. But you must habitually persevere, playing the moves in your mind’s eye and imagining the aftermath and its opportunities: think of the board without White’s knight and (in effect) without Black’s g-pawn, which will have replaced his current f-pawn. Examine every check you then would have—with every piece. There would be three: Qf5; Qxd4; and Rxd4. The first, Qf5, is met with QxQ and nothing more. The second, Qxd4, causes Black to play QxQ and then White to play RxQ+; the king moves, and in the end White has traded his knight for two pawns. But Rxd4+ is another matter. If Black moves his king, White plays RxQ. If Black plays QxR, White plays QxQ. So the original move, Nxf6, is a good one, gaining Black’s queen and two pawns in return for a knight and a rook.