World class table. You're in 3NT. Say a small ♥ is led and third hand plays the ♥J and you have two stoppers in it, the A&K.
You know to fulfill the contract you have to duck it once. But there's a danger lurking that if the defender is astute,which he is might shift to another suit in which you have only one stopper. At the same time it's not all cut and dried that the defender can count your tricks so add up and shift. On right defence the contract is always down. So is it better to duck or not to? You may have only one stopper in the suit led and two in the "other" danger suit which the defender isn't sure of. The bidding was 1NT 3NT. Basically is it as difficult for a Wclass defender to smell the shift as it for say an expert level given that the declarer is also Wclass so will mosdef know the pros and cons of the action he would take.
Plus... it's free!