Source: DEFENSIVE FALSE CARDS & COMBINATIONS By Maritha Pottenger

A basic principle of defensive play is: when it does not give anything away, play the card you are known to hold as soon as possible!! (Example: You have KJ10 and Dummy has AQxx. Declarer takes the finesse to the Queen and then cashes the Ace. Play the KING under the Ace. (Your Jack and King are equals once the Queen is played.) You are known to hold the King. Declarer does NOT know who has the Jack.

Another principle is: when Declarer learns something about your hand, try to inform partner as soon as possible. (An example would be revealing your distribution or high card strength to partner when Declarer has gained that information.)

Be stingy about revealing information to Declarer. This is why, when you have a long suit that Declarer and Dummy can both ruff, it is often best to make all your discards from that long suit. That does not give away any extra information to Declarer. Discarding in another suit would give away more data.

Remember: the card you lead in the middle of a hand (when you can see Dummy) may be different than the card you would have led as an opening lead.

Train yourself to recognize mandatory “false card” situations (when you play an unnecessarily high card to fool Declarer—and to help protect partner sometimes).

Examples would include:

1) Playing the Queen from Q10 when Dummy has J9xx and you strongly suspect Declarer has AKxx.
2) when Declarer play a low card from Dummy which has Jxxx and plays the Queen, assume partner has the King. If you hold 109x, play the 10 or the 9 on the first round of the suit.
3) When you hold K10 and dummy has J9xx and Declarer plays a low card from Dummy, play the King!
4) When Declarer is playing a side suit in Dummy that has AKJxx and takes the Ace and ruffs a low card, if you hold Qxx in front of Dummy, drop the Queen on the 2nd round. (Declarer will think s/he can only get 3 tricks in the suit.)
5) In a trump suit, when you hold K108x and Dummy has AQxx and Declarer plays low to Dummy’s Queen, play the 8! (Now Declarer might return to hand and play Jack to try to “pin’ the presumed 108 in your hand. If you take King first round, Declarer will cash Ace 2nd round and finesse you out of your 10 on the 3rd & 4th rounds.
6) Falsecarding from the J9 is often effective, e.g., Dummy has Q8x and you have J9. If Declarer leads low from Dummy and you insert the Jack and Declarer plays King and partner takes Ace, Declarer is likely to finesse Dummy’s 8 into your 9 on the 2nd round!
7) When Declarer is known to have a long suit and leads a singleton from Dummy towards that long suit and plays King, play the Jack or 10 from J10x. If Declarer has KQxxxxx and partner has Ax, your falsecard is essential to present Declarer with a losing option (leading the Queen to “pin” your Jack next round). If you play low, Declarer will play low next round hoping for a doubleton Ace.

The Encyclopedia of Bridge has an entire section on false cards for further reference and examples.

Be ready to make a “surrounding” play. When dummy has an unguarded 10 (to your right) and you have KJ9 or AJ9, it is usually correct to play the Jack (“surrounding” Dummy’s 10 and pinning Declarer’s possible Queen). Do, however COUNT. If Declarer’s queen could be doubleton, you do NOT want to do a surrounding play. When Dummy (to your right) has the unguarded 9 and you have the Q108 or K108, it is usually right to play the 10 (“surrounding” the 9 in dummy). Although tougher to see, these surrounding plays can also be made by the partner to the left of dummy. That player must be able to visualize an
unguarded 10 or an unguarded 9 in Declarer’s hand and play the Jack or 10 from their own hand.