We always are alert for pieces on the same line, and so have no trouble here seeing the kernel of a discovery for Black on the fifth rank. If his knight could make a substantial enough threat, he could play QxQ a move later. Since the knight can’t give check, think about other threats it could make: is there a square it can reach from which it would then be able to give a check with interesting implications? Yes; with Nxe4 Black unmasks QxQ, and also moves the knight into position to take White’s rook with check on the next move. And Nxe4 does more: it puts the knight into the path of Black’s queen, creating the kernel of a second discovery by Black on the e-file; if White retreats his queen, Black thus plays NxRd2+ and then QxRe1 after White moves his king. To prevent all this, White presumably will respond to Nxe4 with QxQ. Black was ready to allow this because his queen has protection from the bishop at g7, but notice the importance of not recapturing right away. Exhaust your checks first: after White plays QxQ, Black picks up White’s rook with NxR+; then, after White’s king moves, there still is time for Black to play BxQ or d6xQ (the priority of check).