Figure[White to move]

You have a possible capture in RxR; to make it profitable you would need to eliminate the Black rook’s guard—the king. This you do with a check. Bb3+ doesn’t work, of course, because Black can interpose a pawn on c4. But Bh7+ requires Black to move his king, and when he plays KxB it drags his king away from the rook on f8. Now White has RxR, winning the exchange. (If it’s Black’s turn to move, he does the same thing.) Notice that since only the bishop is being sacrificed for the sake of the decoy, the range of targets that makes the sacrifice worthwhile is a little larger than it was when a rook had to be thrown away.