They say black has to make a mistake for it to happen. So I'm guessing white promotes its pawn to a queen, after moving the king to a position where it can defe

They say black has to make a mistake for it to happen. So I'm guessing white promotes its pawn to a queen, after moving the king to a position where it can defend the new queen. The mistake black makes is trying to get its king out of the morass as soon as the pawn moves, instead of just capturing the pawn with its bishops.