Again the key to the position is to study the posture of Black’s king (no freedom of movement) and the lines leading from it in search of pins. Here White’s bishop pins the pawn on g7. Ask what the pawn supposedly protects—not only what pieces, but what squares. The pinned pawn here is supposed to protect f6. The square therefore is loose, at least for the moment. White tries planting a piece on it, like the queen, and determines that Qf6# is mate. Here as elsewhere we are spelling out the thought process, but of course in practice the insight should be nearly instantaneous: the g7 pawn is pinned; the queen mates on f6.