Our studies so far in this chapter mostly have neglected the bishop, making the back rank mate seem mostly a pattern executed with heavy pieces. But the bishop can fill several large supporting roles. The first is a variation on the pattern just considered: once an enemy rook is drawn forward from the back rank with a sacrifice, it may become pinned on its new square by one of your bishops and thus make a back rank mate possible.
The pattern can arise naturally after the enemy castles because his rook then will be right next to his king; this means that if the rook steps forward one square to recapture it will be aligned diagonally with the king and prey to a pin. It’s best of all if the bishop that will impose the pin already is in place, since then you can follow up with a back rank attack without any loss of time.
In the example to the left White has a bishop on the diagonal running toward the Black king’s position on g8; the bishop already pins the pawn on f7. White’s standard sacrifice on the square in front of Black’s rook—Qxf7+—forces the rook to step forward with RxQ and so to pin itself to its king (if Black instead moves his king, then of course White mates with QxR). Now the rook is immobilized and Black’s back rank is defenseless against White’s Re8#.