White King goes to a8, only using white squares and b8. Black has to make the mistake to leave b8 unguarded for one move. Then white pawn to c7. Black King cann

White King goes to a8, only using white squares and b8. Black has to make the mistake to leave b8 unguarded for one move. Then white pawn to c7. Black King cannot b7 because of the white King on a8. Black bishop has to take the pawn, or pawn c8Q will be mate. Black bishop on c7 however is forced stale, since the white King cannot move.