When you know you have to lose at least one trick in a suit that includes the ace and king, you need to lose that trick early by playing low cards from both your hand and the dummy. This is called ducking a trick.
When should I not play my honor cards?
In contract bridge, to duck means to deliberately play low to a trick which you have led and intentionally losing in order to set up a suit or to preserve controls or entries. When applied to a suit you have led it is referred to as a duck but it is called a hold-up when you purposely play a low card in a suit played by the opponents when you could win the trick.
If the dummy has a five-card suit headed by the ace and the king facing three small cards, you want to create two extra tricks using a ducking play.
The opponents have five spades between the two hands, including the spade queen and Jack. You must sooner or later lose a spade trick no matter what, so try lose it as soon as you can while you still have control of the suit
♠ 6 5 3
♠ Q 10 9
♠ J 8
♠ A K 7 4 2