Black’s knight on f5 is pinned to his queen. It is attacked more times than it is protected, and Black has run out of ways to add more defenders to it. The queen has no checks or other scary threats to make to break the pin. But there remains another resource to consider: threats with the pinned piece. There is a check to be had with Ng3, but you see that this loses the knight to h2xN; and this time Black has no compensating capture to make with the screened piece because White has protection for the piece administering the pin (thus Black’s QxB would be met with QxQ).

Threatening the enemy king remains a good idea, but Black needs a way to do it with the pinned knight that does not allow it to be taken. The key is to look for other Black pieces already trained on squares adjacent to White’s king. Black has one bishop attacking g1 and another attacking g2. Can his knight safely and usefully add an additional attack against one of those squares? Yes, with NxB: now the knight bears down on g2, and Black threatens Bxg2# if White takes Black's now-exposed queen. White instead needs to take a defensive measure like Bf3.