Find the kernel of the discovery for Black. His knight masks his queen, which otherwise is aimed at White’s loose queen. (The knight also masks the rook on e8, which lacks a promising target. When you have a knight in the center, be mindful of how it may mask attacks in all directions.) If Black’s knight could give check, he could play QxQ; but it can’t. Nor can the knight create a mating threat, nor can it capture anything. So consider whether it can make any other good threats. None of White’s other pieces are loose, but the rook on f3 would make a nice target for a knight regardless of whether it’s protected. So play with a move that attacks it: Ng5. White finds both his queen and rook attacked. If he moves one out of harm’s way, the other gets taken. He thus would play QxQ, a luxury permitted by the fact that Black’s unmasking piece didn’t give check. But now Black doesn’t play the recapture h7xQ; he plays NxR+—and then, after White captures on f3, Black has h7xQ, winning the exchange with the sequence.
This is another of many examples of taking advantage of the priority of check—the requirement that checks be addressed regardless of what else is happening on the board. Before you play a natural recapture, consider whether you have other damage you can do first with a check, postponing the recapture until a little later. In this case the discovery only turns a profit because Black can take White's rook with check.