White's rook attacks Black’s bishop. What guards the bishop? Black’s king. Does White have any checks that will force the king to move? Yes, if you remember to consider the work your pawns can do as well as your pieces—and if you bear in mind the pawn's ability to change files by capturing. White plays g4xf5+, and now Black must move his king: he can take White’s pawn with it or he can move the king elsewhere, but in no event can he prevent White from winning the bishop with RxB next move.