A minor variation. White has the familiar pin of a knight—here on f6—to Black's queen. This time the pinning bishop isn’t loose—yet; but we still ask whether the knight, by moving out of the way, might be able to create pressure on the enemy king. Once more a bishop attacks the enemy f-pawn. Can Black’s knight add pressure to that square? Yes, with NxN: now two Black pieces are attacking f2. Is it a mate threat? Follow it out, looking for checks and asking what comes next after each White reply. It works like the previous problem: Black plays Bf2+, White moves the king to e2, and then Nc6-d4 is mate for Black.
The point of analyzing the mate threat, of course, is just to determine that if Black starts by playing NxN, White can’t afford to reply BxQ. He has to play d3xN, ending the mate threat but then losing a piece after White takes the bishop with his queen. The initial discovering move removes the bishop’s guard, leaving it loose after all.