False cards, when done properly by Declarer, can mislead the opponents. Your objective is to create a believable (but incorrect) picture of the hand in the mind(s) of the defender(s). An example hand was Hand #19 on August 22, 2018 at Adventures. You held ♠AK986 ♥72 ♦87 ♣QJ76. West (on your right) opened 1♣ and you appropriately overcalled 1♠. East (on your left) made a negative double and partner raised you to 2♠. (You are white; opponents are red.) West bids 3♥. You pass. East bids 4♥ and your partner now raises you to 4♠—which is duly doubled by West.
First, before Dummy comes down, you should expect the spades to divide 2-2. (Partner is expected to have exactly four spades. With five, she would have bid 4♠ immediately and with only three she would not be sacrificing.) The opponents have freely bid a vulnerable game, so West's double is part of the forcing pass system—he must double to tell partner he has two quick losers in spades,** to make sure that East does not bid on without a singleton or void in spades and a really good hand.
The lead is not a heart as you expected, but the ♣5. You are looking at:
♠10542 ♥863 ♦A42 ♣K92
♠AK986 ♥72 ♦87 ♣QJ76
You play low and West plays the ♣A. Do NOT PLAY YOUR the ♣6. You should know that East has led a singleton club, and you must try to fool West and prevent a club ruff.
When West takes the ♣A, drop the ♣J under it. (You don't need to save the ♣J because you can ruff your fourth club in Dummy if need be.) You are trying to create an illusion for West. If East happens to have the ♥A (or ♥A and ♥Q), then West may believe that East did NOT lead a heart because he did not want to lead an Ace without the King—even with both of them bidding the suit. By dropping the Jack, you make it APPEAR that East has led from the ♣Q765—and bidding to game with a double fit only makes sense.
The false card is your best chance to avoid the club ruff. If you can do that, you can be down only one, doubled. West, holding ♠J3 ♥KQJ10 ♦85 ♣A10843, may shift to a heart next. If he plays two rounds of hearts, you are home free. However, he SHOULD try the ♥K (promising the ♥Q) and watch partner's spot card. East will give a totally discouraging heart, and West should then figure out that the club was a singleton. West can shift back to a club; East ruffs and returns a second heart; West leads another club. Then you are down three, though it is still a good sacrifice against their game. However, if West is not sufficiently alert, he will cash two hearts and then shift to a diamond, looking for that fourth trick and expecting partner to produce the ♦KJxx. Then you escape for down only one.
**If you want a handout on Forcing Pass issues, email Maritha or check La Jolla website for handouts.