Figure[Black to move]

The Role of the Bishop III: Driving Back the King.

We have seen two useful roles a bishop can play in creating a back rank mate: it can pin a defender of the back rank after it has been pulled forward with a sacrifice, and it can provide cover for a heavy piece that lands next to the enemy king so that the king can't capture it. Now consider still another use. A bishop can attack an enemy king and thus drive the king further back behind its pawn cover where it can’t reach a heavy piece that lands on the back rank.

The position to the left may not look like the stuff back rank mates are made of, but watch how that changes if Black moves his bishop to d4. The move forces White’s king back to h1, leaving the f1 square unguarded; the movement of the bishop also leaves the Black rook with an open line to f1—so Rf1 is checkmate. And of course you also could have seen this by noting the kernel of a discovery for Black on the f-file and asking what the pieces involved can do to White’s king.