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Expert defense involves collecting clues about the hand from the bidding and early play and putting all the information into place to solve the puzzle of how to defeat their contract.
Take time before playing to trick one, because you have a lot of work to do!
a) Review the bidding, and note whether declarer has shown/denied 4+ cards in any suit(s). Make a mental picture of declarer’s distribution.
b) Make an estimate from the bidding of approx. how many hcp the declarer has, Add your hcp to the hcp in the dummy and your estimate for declarer. This will tell you within a couple of points how many hcp your partner has, and help you to plan the defense.
c) Analyse the opening lead. If partner leads an honor card, you may be able to place the remaining honor cards in the suit. If partner leads a spot card, is it a high spot from shortness or low from length? Ask yourself how many cards in the suit are left for declarer. Vs suit contracts, when partner leads a spot card in an unbid suit, there is a strong inference that he doesn’t have an honor sequence in another unbid suit. You can use that inference to start placing the high cards in declarer’s hand.
d) know your trick target: how many tricks do you need to set the hand?
e) always count the declarer’s tricks. This will tell you what you need in partner’s hand to set the contract.
Trick one signal: Primary signal is attitude (continue or switch) except give suit preference when no more tricks are cashing for our side. The most common situation for this is when dummy has shortness.
Example: They reach 4
after 1NT opening and texas. Partner leads the ace of hearts and dummy has
AQxxxx, 
x, 
Qxx, 
Qxx
You hold: xx, 10xxx, AKJx, xxx
Play the heart ten, suit preference for the higher remaining suit. A diamond switch is necessary, because declarer’s hand is KJx, QJx, xxx, AKJx
After trick one: spot card leads are attitude. A low card says you have an honor here and would like the suit returned. A high card says you want partner to shift to another suit.
These deals are based on hands from “Kantar for the Defense, Volume 1” and “Kantar for the Defense, Volume 2” both by Edwin B. Kantar. I highly recommend this series of books for learning expert defense.
South leads heart jack. North can see she has heart spots strong enough to overtake the jack with the queen and continue with the heart ten smashing dummy’s nine. Key point: don’t leave partner on lead to continue a suit when this is something you can do for yourself.
West leads the diamond jack. East wins the King and needs to figure out how to get 5 tricks before declarer gets 9. Declarer is marked with Qxx of diamonds, continuing the suit will only produce 3 tricks. If West’s side card is spade ace, nothing can be done, but if west has either the heart ace or a club stopper plus the heart queen then switching to a heart will be productive. East plays the heart jack at trick 2 smothering the ten in dummy.