The Defenders' Plan
Defenders must try to give each other as much information during the play of the hand to help beat the contract. This is done via the cards they choose, starting with the opening lead. The picture becomes clearer as each card is played. They are trying to take the tricks to which they are entitled, but, as well as that, trying to deprive declarer of all the tricks to which declarer is entitled. The fact that the defenders get to make the opening lead gives them an advantage, but the disadvantage is they can't see dummy until after they've made that lead!
The defenders develop defensive tricks in exactly the same way declarer does, but it doesn't always look the same because the defenders can't see each other's cards. The techniques they use are Promotion, Length, the Finesse, Using the Trump Suit for Ruffing, and Discarding Losers on Winners, just as declarer does.
(1) Promotion. In no trumps, when a defender is holding ♥KQJxx of a suit, and is on lead, the correct card to select is the ♥K. This shows partner they have touching honours underneath that. In no trumps it's three honours, called a sequence (or nearly three, called a broken sequence) ♥KQJ or ♥KQ10, and in a suit, it could be two honours (♥KQxxx). It's called leading top of a sequence. The reason they do this is to promote their middle cards into winners. They will lose to the ♥A unless they are lucky enough to find it in partner's hand. Remember that for promotion to work, you must lose the lead to turn your middle cards into winners later.
(2) Length. In no trumps, when you lead a low card from a long suit in which you do not have a sequence, you're trying to establish long suit tricks. Fourth highest (from the top down) is the most popular strategy. You're expecting declarer to win an early trick or two in the suit, and you will win the later, long suit tricks, when the other side has none left. For this to work, you must regain the lead when your long suit cards are winners. e.g. you hold ♥A7532. The correct card to lead is the ♥3 (fourth highest). You expect declarer to take a trick, but you want to make your long suit tricks later.
(3).Finesse. The defenders are also trying to take finesses, when someone leads an A from a holding like ♥AKJxx , and they need to know where the ♥Q is. If they can't see it in dummy, it may be in declarer's hand or partner's. It's not a good idea to play the next winner, the ♥K, until you receive a signal from partner to tell whether they have the ♥Q or not. If they don't, it would be better to lead another suit, and keep your ♥KJ sitting over declarer's ♥Q.
(4) Using the Trump Suit. The defenders are entitled to win tricks by trumping just as declarer is. If they lead say AK of their long suit, and their partner shows them, by the cards they play, that they have a doubleton, then a third card will allow partner to ruff, assuming declarer had a winner left in the suit. And the fifth technique of discarding losers on winners is available too. For this to work, you need top cards, and a different number of cards between the two hands. e.g. ♣AK4 in one hand, and ♣Q7 in the other.
Finally, there's no substitute for using your own logic in the defence. Carefully looking at what's in dummy, and also noting the way declarer is playing their suits will reveal a lot about what's in their hand!
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