White’s pawn on h3 is about to take Black’s knight. The easy thought is to retreat the knight, but of course that would be rash. You see that the knight and Black’s queen both are aimed at h2, and indeed that Qh2 would be mate if White’s knight on f3 were not defending the mating square. Black has no way to capture the bothersome knight, but perhaps he fruitfully can threaten it. Now of course attacking a knight with another piece generally will not frighten your opponent, particularly in a case like this where the knight is defended. But it’s different when the attack is a fork, such as Black’s Nc6-d4—attacking the White knight and queen. A knight fork against a fellow knight usually is no good, but here it is as good as forking the king: White absolutely cannot afford to play NxN, as it permits mate on the move for Black. Yet if White tries to save his queen, as with Qd1, Black plays NxN and mates a move later. We thus arrive at a familiar outcome: White’s only recourse is to let go of the attacked pieces and take other measures to defend against mate—here, by playing h3xN. Now Black wins the queen with NxQ.