Black’s king largely is boxed in; the question is whether the burial can be made complete. When White inspects his checks he sees that Nh6 attacks not only the king but also g8, while g6 would be attacked by his queen and bishop. It’s very nearly a smothered mate—except that White’s knight gets taken by the bishop on g7. White has recourse, naturally: he takes the bishop. Thus 1. QxB+, requiring 1. …NxQ; and now Nh6 still mates because although White’s queen is off the board, his bishop continues to seal off g6.