Where does White have a possible discovered attack? On the side of the board, where we find the familiar kernel of bishop masking rook on the same file. There is an easy target: again Black has a loose rook, this time on a7—a valuable enough piece to justify sacrificing the bishop if necessary. White looks for a check the bishop can give as it moves out of the way. He finds Bc4+, a nice backward move that wins the loose rook without need of a sacrifice.