The defender places the king opposite the attacking king and pins the bishop along the file between them. The pin paralyses the attacker's coordination, and when the white king steps aside the rook checks and returns.
The Cochrane defence is the most reliable method for the defender in rook-and-bishop versus rook when the king has been driven to the edge. The pieces stand in a line: black king on d8, white king on d5, white bishop on d4 pinned by the black rook on d1. As long as the bishop is pinned, White cannot use it, and the rook on a7 alone cannot mate.