It often will happen that Black has a piece defending g5 and ready to take your knight if it lands there. In that case you still may be able to use the Bxh7+ sacrifice to force a sacrifice by your opponent on g5 to stop your mate threat. Here White has the key elements of the pattern in place: a bishop aimed at h7; a knight on f3, able to attack the seventh rank in one move; and a queen able to get to the h-file in one move. (These are common positions for those three pieces, which is why this mating idea arises a lot.) But the g5 square is guarded by Black’s queen; that means the mating idea won’t work—but that Black will have to pay to avoid it: 1. Bxh7+, KxB; 2. Ng5+, Kg8; 3. Qh5, QxN; 4. QxQ. (White’s second and third moves can be reversed without consequence.) White wins a queen and a pawn for a bishop and a knight.
Notice that if Black starts with a bishop behind his queen on d8 (or with the queen behind the bishop—it doesn’t matter for these purposes), the sequence no longer works for White: g5 then is attacked twice but guarded by White only once. But then if White had his h-pawn advanced to h4 it would work again after all, because White’s knight then would have an inexpensive second layer of protection against Black’s two more valuable attackers.