White’s knight and Black’s queen and rook are on dark squares; White would like to play Nh7 to fork them, but then Black’s king can capture the knight. So what then? Consider the position that results. When the king or the pieces around it move, it’s always wise to begin by checking for any pieces that may have become pinned—pieces, in other words, that now find themselves lined up with the king and that can’t move without exposing it to attack. Here the king’s move to h7 would cause the pawn on g6 to become pinned. As we know from earlier studies, a key consequence of a pin is that the pinned piece can't guard anything because it can't move. In this case the pin of the g6 pawn means the h5 pawn—and perhaps more importantly the h5 square—are left loose. Perhaps you can take advantage. Carefully examine any check that could be given from there. Think of Qxh5+. The king would have to retreat to g7 or g8. With the rook behind it, the queen then can move in for the kill: Qh7#.

So the original knight fork Nh7 does work, winning the exchange; Black can't afford to play KxN, and so must move his queen and let his rook be taken. But it only becomes possible to see this once you imagine what would happen if the fork "failed" because your knight got taken.