We start with an open diagonal to Black's king, and a Black piece—the bishop—on the same line. Now look for checks White can give with his own bishop, and find Be5+. If the Black bishop could be swapped out for a better target, White would have a nice double attack. So capture Black's bishop with another piece: 1. RxB, QxR and now the scene is set for Be5+, forking Black’s king and queen. The point of seeing the position here is that since the queen is the target, White’s bishop needs protection on the forking square; it gets it from the f4 pawn. One of the many nice things about having a pawn in or near the center is that it can provide a protective anchor for a piece delivering a fork.