At present Black has no forks; only one White piece is on the same color square as his knight. It might be natural for Black to consider Nh3, creating a threat of mate next move with Qf2 or Qg1. But then you think about what checks White would have in reply and notice Qc8+—a queen fork that would win Black’s knight on h3. So Black scratches that idea and starts by examining every check of his own. There are three: Qd1, Qf2, and Qg1. Qd1 loses the queen without accomplishing anything. Qf2 forces White to reply KxQ and almost leads to a knight fork at d3, but the square is protected by White’s bishop. Qg1 also forces the reply KxQ, but it moves the king to a different dark square. Now Black can launch a fork from e2, which is occupied but unprotected; NxB+ thus wins the queen a move later.