When you have a suit that doesn’t currently have sure tricks, you may be able to promote tricks. You promote tricks by getting the opponents to take their sure tricks, leaving you with sure tricks. Here’s a small example:
Here you don’t have any sure tricks in Spades, but the opponents can only defeat one of the K, Q, or J with their ace. Suppose you lead the K which is taken by the A, and then you get the lead back. You now have 2 sure tricks in the Q and J.
Let’s look at another example. Here you have to knock out two winners before you are left with sure tricks.
Here you have the Q, J, T, and 9. The opponents have the A and the K. They can only defeat two of your four high cards in spades. Suppose you lead the Q and it is taken by the A. You get the lead back and lead the J, taken by the K. Your remaining T and 9 are now two sure tricks.
Let’s take a look at a full deal.
Here the contract is 3NT. We have to take (3 + 6) = 9 tricks. We have 7 sure tricks with enough entries to dummy to deal with the blocked diamonds. We should win the opening spade lead and immediately lead a high club. If the opponents take this, we win whatever they return and cash our 8 remaining sure tricks. If the opponents don’t take this, then we lead another high club. If they held off (refused to take) twice, we switch to cashing our other suits. Otherwise we win the return as before, and cash our winners.
Let’s take a look at one more complete deal.
Here we 7 sure tricks. We need 9 for 3NT. We can promote one trick in hearts, and two in diamonds. If we try to do both, the opponents may take 5 tricks before we can get 9, in two red aces and three spade tricks. To avoid this, we should try and promote tricks only in diamonds. Win the spade lead in dummy, lead the diamond queen. Let the opponents take the first or second diamond and win their return. Then cash your 9 tricks. If the opponents let you have 2 diamond tricks, just cash your 7 sure winners. Don’t lead a diamond. One opponent could cash 4 more diamonds and the ♥A for a total of 5 tricks.
Advanced points
Here’s an important point about unblocking. In the hand above, we had two entries to dummy, so it didn’t matter where we won the spade lead. Suppose the ♥K were in hand instead of dummy. Then you would have to be careful about entries.
Here we can still follow the same general plan as above, but more carefully. We win the opening spade lead with the ♠K in hand, cash the ♦AK, and only then lead toward the club K. Now if the opponents return a spade, we can win in dummy for the last time. While we are in dummy, we can cash the ♦Q and then the rest of the winners in our hand.
It’s true that we could try and lead the ♣Q and ♣J first to try and get the opponents to capture them and leave the ♣K in dummy as an entry, but if East has 3 clubs and the ♣A, he may hold off to try and keep us out of dummy. It would be unlucky for east to have this, but it’s better to make our own luck by playing carefully.