White has two pieces trained on h7, almost poised to execute a classic bishop-and-rook mate with Rxh7. Black understands this, and so has posted a knight on f8 where it can protect the mating square. The knight in turn is guarded by Black’s queen, but the fact that it is protected is itself of no consequence; the important question is whether the guardian would be able to perform the same defensive services as the knight. It would not. Thus White plays QxN+, and after the reply QxQ Black is unable to stop Rxh7#. Indeed, Black is unable to prevent checkmate no matter how he replies to QxN; the most he can do is delay the execution by a couple of moves with Kh5 (to which White responds Rxh7+), then Kg4 (resulting in White's Qg8#).