South leads the 9.
East to guarantee a contract of 6.
Declarer wins the diamond in hand and must lead the Q; the defense ducks in order to prevent an immediate claim. Declarer now tries the J (or ace and jack); again, a defender with four or more hearts including the queen must duck, or twelve tricks are made available. Now declarer plays on clubs, setting up the slam-going trick in that suit.
Trap: if declarer plays both top spades, the defense could duck. Then, when coming on lead with another suit, the K could potentially be cashed for the setting trick. If instead declarer plays out the hearts or clubs first, the defense can grab its winner, as there will not be enough tricks.