Declarer has the built-in advantage of full awareness of his available resources; a disadvantage is that the defenders can see half of them. As declarer, you need to hide the rest of your cards as best you can.
PLAY THE CARD YOU ARE KNOWN TO HOLD
Most declarers handle this one automatically. Winning a trick with your highest of equal cards generally is best. If an opening lead runs to RHO's Jack, you win the King from KQx, thereby temporarily concealing from LHO the location of the Queen. Had you won the Queen instead, LHO would know that you also have the King; otherwise, his partner would have played it. Similarly, with AKxx, winning with the Ace can leave in doubt the location of the King.
Sometimes the choice isn't so clear. Suppose that you have AQJ in your hand. LHO leads the Ten of that suit against a Notrump contract, and RHO plays a spot card. From RHO's perspective, the lead could have been from a variety of holdings, depending upon their agreements:
Tx T9x JTxx QT9x KT9x AT9x KJTx
If they play Rusinow, then you know that the lead must be from shortness. Win the Queen; neither opponent will know where the Jack is. If they play Zero or Two Higher, win the Queen. RHO knows that you have it.
If they play Standard, then the lead must be from KT9x. Win the Jack; RHO already knows where that one is, but wait. That tells LHO that you have the Queen; otherwise, his partner would have played it. So win the Queen, but wait; now, RHO knows about both the Jack and Queen. Does that matter, though? No, he will return the suit regardless; so play the Queen. It is LHO that must be kept in the dark; he might play his partner for the Jack when he gets in.
PLAY THE CARD YOU ARE NOT KNOWN TO HOLD
Many players suffer from what I call the Pinochle Syndrome. In that game, it tends to be necessary for players to cash every available winner as soon as possible; so they do. Needless to say, that frequently is not the winning defensive strategy on a bridge hand. Average Jane, however, has other ideas. She can be relentless in her hunt for winners in partner's hand. When one avenue fizzes out she will try another, often with little regard for the potential consequences.
Sometimes you can help the defenders to pursue their quest for quick winners, by showing them something that you have. Example: Playing in a suit contract, you hold AKx opposite xx in a side suit. When that suit is led to your ace, it might also be best to cash the king as well and show the defenders that there is no future there. When an opponent obtains the lead, she might well break a key suit, which is just what you wanted all along.
FEIGN DISINTEREST IN KEY SUITS
This favorite ploy is worth a lot of tricks in the long run. The idea is to make discards in a suit in which you are interested, rather than somewhere else. When trumps are being drawn or a long suit is being run, at least one of the opponents is under pressure to make discards. The more cards he sees played in a suit, the less likely it is that a discard in the same suit would be costly.
When discarding from dummy, if you want the opponents to throw away diamonds, then discard a diamond or two. Holding Axx opposite Kxxx, and you want three tricks without losing one, discard down to Kx in dummy. Try not to be too obvious, though. If three discards must be made, pitch a diamond, then a heart, then another diamond. Try to make it look as if there is some uncertainty in your choices.
Discards from your hand are even more difficult to read. It generally is right to rid yourself of the highest spot cards you can afford, feigning shortness there. Holding AK952, for example, throw away the deuce and Nine of that suit; there is a good chance that the Five-spot will be a winner at the end.
Why not keep the deuce? Someone might actually remember that it still is missing. It's the middle spot-cards that are easiest to forget.
PLAY SECOND-HAND HIGH!
This is one of my favorite "secrets", because it seems little-known even by a lot of better players and is most likely to succeed against them. Occasionally, this tactic can conceal information about declarer’s holding for a while. In the following examples, LHO leads a low card:
(1) If you play the Jack and take the Queen with the Ace, LHO will not know where the Ten is. If you play low and let RHO insert the Ten, then LHO will know that he can safely underlead the King later if desired.
(2) LHO will learn that his partner has QJT no matter what you do. If you play low from dummy, though, LHO also will learn that another trick can be cashed by either defender. Why? Because if RHO held the Eight, he would play it. Call for the Nine immediately, then grab the Ace; LHO will not yet know whether you hold another card in the suit. Similarly, playing high from Txxx or even Jxxx in dummy can temporarily conceal one or more of RHO’s cards from his partner.
(3) Play the Queen. If it is covered, LHO will not know where the Jack is; and RHO might not know either, depending upon LHO's choice of leads. Failure to make this play will tell both opponents immediately that you have no loser in the suit; and that's a fact worth hiding.
Note that in those three examples, there would have been no benefit to playing high from dummy unless there were a lower-ranking card in your hand; for then there would have been nothing to conceal.
(4) Play the Queen. If it is covered, then once again at least one defender will not know whether you have a singleton. They might well guess that you do have another card in your hand; why else would you have played that way? If the Queen is not covered, then the cat is out of the bag anyway. This time, you are concealing not a card, but your distribution.
Another type of ostensibly meaningless cover, however, compels RHO to make a decision with incomplete information:
T92 53 QJ9864 AK
(1) When the Five is led against a notrump contract, play the Ten. RHO, not knowing who holds the Three, probably will cover to prevent you from winning a cheap trick if you hold AK3. Now, should that player obtain the lead right away, he cannot be sure whether it is safe to continue the suit — at least, not from the play in that suit itself; the defenders may have other tools available.
Finally, a nonsense-play can prove useful. With a small singleton opposite Qxx in dummy, play the Queen, but only if you want the suit to be continued. Remember that the weaker dummy's suit is, the more likely that it will be led.
Naturally, you might need to avoid squandering dummy's high card if it could jeopardize the holding in your hand, such as by compromising a stopper. When holding ATx opposite Jx, for example, one would not want to play the jack right away if subsequent control of that suit were important, which it usually is.
REVEAL NOTHING UNTIL YOU MUST
Every lead of a suit allows the defenders to communicate something to each other, either regarding that suit or something else. That is the nature of the game, and that is why you want to avoid playing on any key suit until you must. With a side suit of Kxx in dummy and AQxx in hand, for example, avoid playing on that suit as long as possible; that makes it more difficult to count your distribution and place the honors.
A common failing in this area is the situation where declarer has some high trumps, plus a winner and a loser on the side. Hoping for a discarding error, declarer leads out the trumps; but wait. Average Joe virtually never leads the last trump right away; he cashes that side ace first, which serves only to help the defenders. A.J.'s chronic unholy fear of prematurely playing off the last trump holds true in this scenario. As declarer, you can do better.
This brings us to another of my greatest "secrets" — one which even a lot of good players have overlooked:
If you lead a trump from Axx in dummy and insert the Ten from KQJT, you have just told RHO that his partner has no higher trump. By winning the Jack or Queen instead, there is some doubt about the actual holding. The Jack is best; for now LHO will not know who actually holds the Queen. From her perspective, that play might have been a finesse.
Observe this phenomenon for yourself. The next time your declarer ruffs with a Jack, he will not have the Ten. If he leads to a Ten, he will not have the Nine. When running a suit, he never leads his honors in an order such as Queen, King, Jack. He invariably will have played his sequence either in top-down or bottom-up fashion, especially in trumps. Watch and learn; but don't tell.
"It's not what they could, it's not what they should, but it is what they would."