White's bishop on b5 attacks one of Black’s knights, which is guarded by Black’s other knight and his queen. Meanwhile White’s queen attacks the other knight on f6, which is guarded—again—by the queen and by the first knight (the one on d7). The key observation is that Black’s queen is guarding both pieces. This suggests that the queen is overworked. Normally we might take advantage by capturing one of the pieces the queen guards, but here that won't quite work because of the extra protection those two pieces have; whichever knight White captures, Black recaptures with his other knight and avoids trouble. But we still can exploit the overworked queen. The trick in this case is to throw another attacker at one of the pieces the queen guards. White does it with Bg5, which also pins the f6 knight to the queen. Notice that even with the pin, each of Black’s knights is protected as many times as it is attacked; but Black nevertheless is in trouble because the queen is doing too much work. If White next were to play BxNd7, Black would have to recapture with QxB—and now the knight on f6 would be loose. Black's likely reply to Bg5 by retreating his queen, say to f8. Then White plays BxNf6 (not BxNd7, which allows Black to escape with no loss after he recaptures). Black recaptures NxBf6, and now White wins a piece with QxN.