White's c3 knight masks his queen, which otherwise could play QxQ. But for a discovery like this to work, the knight needs to threaten something even more valuable than the queen—viz., the king. Yet how? The knight is on c3, very far from the Black king on g8. Still, examine the king’s position closely. White’s bishop on h6 cuts off the king’s escape squares entirely; if the king were attacked it would have nowhere to go and might be mated. White can’t attack it now, but might he move the knight to a position from which it could attack the king a move later? If so, that move might be as menacing as a check, given the constraints on the king’s mobility. Here the knight can move toward the king with Nd5—and a move later White then would be able to play Ne7#. Nd5 therefore is as effective as a check; Black must parry the threat or the game is over. But then it pretty much is over in any event because White plays QxQ.

This position is a nice study in how even a knight that seems very far from the enemy king may have the power to make a mortal threat against it. It also illustrates the importance of watching the vulnerabilities of the kings at all times, no matter how distant they may seem from the rest of the action.