Figure[White to move]

Here is another illustration of a by-now familiar point. Black’s king looks secure if you focus only on White’s queen and rook; if White plays 1. QxQ+, Black replies KxQ and then his king defends e8. But if you are attentive to all of your resources you see the knight powerfully placed on g4 and able to give check on h6 or f6. The f6 square is guarded by Black’s knight, but h6 is more vulnerable. 1. Nh6+ would require Black to either move his king to h8 or to play g7xN. Consider both:

(a) If Black plays Kh8, his king has been driven back from f8 and the queen that sits on that square; so now White mates with QxQ.

(b) If Black instead plays g7xN, this opens his pawn cover and invites consideration of a check on the newly available file: Rg3+. This forces Kh8, as Black’s queen is pinned. Now again White mates with QxQ.

It’s another study in the variety of ways a king can be driven into a corner so that it can't participate in its own defense.