Black is threatening Qxd4, a safe, close check; no response White could make would avoid trouble. (One possibility would be 2. Ke1, Qh4+; 3. g2-g3 (interposing to block the check), Qe4+ (queen fork of White’s king and rook, taking advantage of the line opened by White’s pawn move)). But it’s White’s turn to move, so he has a chance to forestall disaster. The pin of his knight prevents him from playing NxQ, so he looks for other offensive options and finds the check Nd6—a knight fork of Black’s king and queen. It doesn’t work as a fork because Black plays BxN; but then the pin has been broken and White now is free to play NxQ. Lesson: sometimes a pin can be broken by creating a disturbance elsewhere on the board that requires the piece inflicting the pin to leave its post. Ask whether the pinning piece guards any sensitive squares on which you can make trouble.