Chandler realized that after 54.gxh6+ Kxh6 he will be left with the considerable material advantage of a  and bishop against a bare king. However, since the bishop is unable to control the promotion square h8, Black will draw if she is able to get her king to control h8 due to the wrong rook pawn fortress. But Chandler calculates further, and realizes that it is he who will win control over the h8 square after 55.Kf6, and thereby win the game.
Chandler realized that after a specific move, he would have a significant material advantage with a bishop and a king against a bare king. However, Black could draw if she could get her king to control h8 due to a pawn fortress. But Chandler calculated further and discovered that he would win the game by controlling h8 after his next move.