Here is the same position just viewed but with one change: Black’s queen is on c8. This alters the payoff but not the basic idea. Now after 1. Qe8+, RxQ, 2. Bd5+, Black’s interposition of his rook on e6 is not quite futile; for the rook is protected by Black’s queen. But White still has 3. BxR+, QxB; 4. f5xQ, winning the exchange. The initial move 1. Qe8 performs the same function as before. It forces Black to move his rook out of position so that it can defend against mate only indirectly (by costly interposition) rather than directly (by capturing your bishop).