White’s queen and bishop are pointed at f7; White imagines the mating sequence 1. Qxf7+, Kh8; 2. Qg8#, and sees that it is prevented only by the Black queen on d7. Next he sees that the queen also protects the rook on c6. Time to play RxR? Actually no; for Black’s reply QxR will open the seventh rank and permit the rook at a7 to assume the defense of f7. (The mating square f7 was protected twice; you want to be sure to notice attackers or defenders stacked one behind the other.)

Yet it’s true that the queen is stretched too thin, and as we have seen elsewhere it is important in such cases to consider both move orders: imagine first taking one of the points the queen is trying to defend, then imagine first taking the other. Since 1. RxR doesn’t work, White thus considers 1. Qxf7+. This seems odd because it obviously doesn’t work—as a mate threat. But it does work to distract Black’s queen, which is required to make the recapture. Now White has BxQ+, and after his bishop is retaken (the priority of check requires it) White finally has RxR, winning the exchange and a pawn.