Back to normal positions. Black’s queen and f4 knight both attack g2; Black is prevented from mating there by White’s queen on g4. Black has no good way to drive off White’s queen, but he does have a way to obstruct its protection of the mating square: Ng3+. Notice the pressure the move creates. It’s a check, so it has to be addressed directly, and if White moves his king it gets mated right away with Qxg2. White thus takes the knight with his queen. But then comes h4xQ, and White runs out of ways to stave off mate on g2. He can interpose his rook at e2, but Black’s queen just takes it.