On studying Black’s king you see that it's sealed onto h8 by a combination of its own pieces and the bishop you have on h6. This invites a smothering and you do have a knight in the vicinity for the purpose; but the only available square for it is g6, which Black guards. Often, however, there are two squares a knight can use for a purpose like this; here the other one is f7. Too bad it’s occupied, but then again the occupant is one of your own pieces—so you can evacuate it with a threat: 1. Qf6+, requiring the reply BxQ. Now 2. Nf7 mates. And of course you might as well have seen this by just starting with the check 1. Qf6+, seeing the reply BxQ, and then looking for your next check and spotting Nf7#. It's another case where the king is smothered in part by pressure your pieces put on the squares around it.