>>20979white pawn g6 threatens rook.
black can't move rook because doing so would lead to checkmate, wherever it happens to move. so black bishop f6 protecting the black pawn on g7.
white pawn X black rook on f7
black queen X white pawn on f7
white's now up a rook and black would probably resign....
In the actual game, black pawn X white knight rather than bishop defending the pawn on g2, sort of a throwaway move by black, and then black resigned after check by white.