Bridge Chestnut 9 - "Always play the card which you are known to hold"
The most obvious example of this occurs when you are declarer. You accidentally drop the heart ten and the director correctly tells you to put it back in your hand. The defence now know that you hold that card. Donít play the Jack or the nine before you play the ten.
Hereís another example:-
Declarer plays small to dummyís 9 and partnerís ace. Declarer now knows that you hold the Ten. Declarer regains the lead and leads small towards J74. You should insert the ten. Declarer may now believe that the suit is breaking 3-3 and attempt to establish the 4th trick in the suit. If you had inserted the 8 and the Jack had won the trick, he would know that you hold KT and abandon the suit.
After East makes a jump overcall of 3, South becomes declarer in 4. The defence begins with A, K and Q. Declarer ruffs with the J which holds the trick. East is now marked with the Q. Declarer plays 4 to the Ace on which East drops the Q. Will declarer play this as a singleton? I think so. Now, he finesses into Eastís T for the setting trick. If he had dropped the T under the A, declarer will play to drop the QT doubleton, the 9 calmly collecting the 8