The kernel of the discovery here should be obvious enough: White is poised to give check on the seventh rank by moving the bishop out of the rook’s way. He needs a good target for the bishop. It’s on a light square; what Black pieces are on light squares? None. But that is no reason to give up. White’s task is clear: get a Black piece onto a light square—probably by forcing one of them to move there with a threat. Pawns are best for this purpose, since in the face of a pawn threat a piece usually must move; the threat can't be dealt with by just adding protection to the piece. A threat against a knight is especially useful here because every time a knight moves it switches to a different colored square. So b3-b4 forces the knight to move, and wherever it goes it gets taken—either right away by White’s bishop or rook, or (if the knight moves to c6) by the discovered check Bd5+. After Black moves his king, BxN then takes the piece, which is defended by Black’s rook but also attacked a second time by White’s rook.