With the pawn on the seventh and the defending king far away, the attacker wins by bringing the bishop to the far end of the defender's diagonal, interposing, and forcing it off. The defending bishop needs two diagonals to hold; here it has only one usable one.
This is the classic Centurini mechanism. White's pawn stands on b7 and the black bishop on h2 controls b8 along the long diagonal. The other approach to b8, via a7, is blocked by the white king on b6. So the black bishop has exactly one diagonal to defend b8, and a bishop with one diagonal can be evicted.