Examine how White’s pieces bear on the enemy king; consider especially whether any of them are aimed at the same squares. White’s queen and rook are pointed at a square adjacent to Black’s king: f7. White could play Qxf7+ if the bishop on e7 were out of the way; after the Black king moved to h8, White could follow with QxN#. What this means is that the bishop on e7 is pinned to the mating square f7, and is a ripe target.

How to take advantage? When you have a pinned target, there are two things to do with it: capture it by amassing force against it, taking advantage of the fact that it can't run away; or go after pieces and squares the pinned piece is supposed to protect. Try the first route first. Ask how many times the bishop is attacked and defended, and whether you can make the former number bigger than the latter. At present the bishop is attacked once and defended once. White thus adds an attacker with 1. Bc5. Now Black needs to add a defender, and his only way to do this without losing a piece on the spot is with Kf8. But now—and this is critical—the Black king has moved. When this happens you must consider whether any old pins are gone and any new ones have been created. In this case the king’s move to f8 suddenly causes Black’s bishop to become pinned again, and this time absolutely, by White’s bishop. The observation is crucial because it means White’s queen now is free to add yet another attack to the bishop with Qf6. Black is unable to play BxQ. He also is unable to add any more defenders to the bishop. It falls next move to BxB.

Now notice that Black does have another defensive idea: he could reply to White’s 1. Bc5 with Bf8. This allows White to play Qxf7+, but notice that from its new perch on f8 the Black bishop guards g7 and prevents White from mating there next move (after Black moves his king to h8). Yet it doesn’t help. After 1. Bc5, Bf8; 2. Qxf7+, Kh8, White plays 3. BxB and will mate a move later. If Black plays RxB, White has 4. QxN#; if Black plays 3. ...Re7, White has 4. BxN#!