Black’s knight masks his bishop, but the bishop lacks a good target if the knight gets out of the way. So think about other things to do with the knight. See how it can jump into the path of the rook on the e-file, creating the kernel of a discovery. It appears that the rook would have no target, but remember to ask whether the knight’s move might force useful adjustments by the enemy. What threat would Ne3 make? It would fork White’s rook and bishop. White would want to move the rook out of the way, but if he removes it from the first rank he leaves the bishop loose. To protect both pieces he therefore has to play Re1—and this puts his rook in line to be taken by the the discovered attack Nc4+. Black wins the exchange.