Look at how Black’s rook attacks f1 and would mate there except for the protection provided to the square by White’s queen. When a queen single-handedly guards against mate it can become a vulnerable target in its own right. Targets in turn call for investigation of forks and other tactical strikes to try to win them. In this case Black can pressure both the queen and the rook on g5 with Qd2. From there he threatens QxQ#, and if White plays QxQ his defender of the first rank is gone and Black mates with Rf1. White can avoid the mate by playing Qg1—but then he loses the rook to QxR. The position amounts to a simple queen fork of a loose piece and a key defender of the back rank, but to see it you have to be aware of both ideas.
This is an example of the second pattern—(b)—mentioned in the previous frame. White's queen is committed to the first rank to prevent mate there; the piece therefore is immobile and becomes a good target for a fork. Black just needs to find a loose piece to go with it, which leads him to the rook on g5.