This is similar to the previous frame. What Black pieces are loose? The knight at a5 and the rook at e2. It would be hard to fashion a double attack against the knight, but the rook, isolated deep in White’s territory and near the White queen, is a perfect target. Is there a square White’s queen can reach that would enable it to attack Black’s king and rook at the same time? If the queen were on g4 it would be aimed at both targets. But its path to the king would be blocked by the pawn at g7, and of course the White knight already occupies g4. What White needs is a way for the knight to vacate g4 with a capture or threat that requires Black to respond, and a way to remove the pawn at g7. Nxf6 and Nh6 suggest themselves as ways to achieve both objectives at once. Nxf6 doesn’t quite do it because Black can reply QxN without moving the g7 pawn. But Nh6+ forks queen and king and so requires g7xN in response—after which Qg4+ forks and takes the Black rook.