Figure[White to move]

White has a potent battery, this time on the f-file. Ideally he would like to drop his queen to f8, let it get captured by Black’s rook, then recapture with his own rook and deliver mate. The only worry is an interposition: if Black then plays Be8, the mate seems to be spoiled; for then if White plays RxB, Black has KxR. This is a common difficulty in carrying off a back rank mate. But scan the king’s lines (or the bishop’s vulnerabilities) and you see that the bishop is pinned and can’t interpose after all. So 1. Qf8+ followed by 2. RxR is indeed mate.