Declarer regularly establishes tricks in long suits to make their contract. Once they’ve done this, they often need an entry to reach their winners. To make things difficult for declarer, the defenders mustn’t take their winners until they can break declarer’s communications with dummy. Here's an example:
Declarer plays the ♠K to force the ♠A, promoting the ♠QJ104 for four tricks. If the defender sitting East doesn’t take their A until the second round, declarer won’t be able to reach these winners unless there’s an entry in another suit. The defenders can help each other with a count signal when declarer is playing the suit. This means that instead of telling partner whether you like the suit or not (it’s a suit declarer is playing on for tricks, after all), you tell partner how many cards you have in the suit.
If West plays the ♠4 followed by the ♠5, (playing low high will show an odd number) East will work out that declarer started with two spades, and will take their ♠A the second time the suit is played. If declarer has an entry in another suit, your holdup play will not help, but at least you did your best.
Here if declarer leads the ♠K and you win the first trick with the ♠A, declarer can use an outside entry to get to dummy’s winners and take four tricks. If you hold up, declarer can use the external entry to get to dummy, but when you win the trick, declarer would need a second entry to dummy to get back to the established winners. If there was only one outside entry to dummy, your hold-up play restricts declarer to one trick in the suit. You might think it is dangerous to holdup with an ace in a suit contract, but if there is only one entry to dummy, then sacrificing your ace might be worthwhile.
Here’s another opportunity to make declarer’s life difficult by holding up your winner.
Trying to establish winners in this suit, declarer leads the ♠Q and plays a low card from dummy. This would work if partner held the ♠K, as partner’s ♠Q would win the trick and declarer would repeat the finesse. It would also be successful if you won the first trick with the ♠K, as the remaining cards in dummy would be winners and declarer would have a low card left to get there. If you let the ♠Q hold the first trick, declarer will probably take the finesse again, thinking you do not hold the ♠K.
If declarer does take the finesse a second time, you now win your ♠K. Unless declarer can reach dummy in another suit, you have successfully stranded declarer from the winners there. Your partner helped a lot by playing the ♠8 followed by the ♠2 to show an even number of cards in that suit.
In this similar situation, when declarer plays the ♠Q, your partner should signal with the 2 to show an odd number of cards. If you have nerves of steel, you will NOT win the ♠K! Why? Because it’s odds on that declarer, thinking the finesse worked and your partner is holding the ♠K, will take the finesse again. If you take the ♠K on the first round, declarer has an entry to reach the five winners over there, but by holding up you give them a chance to go wrong.
Another way to make life difficult for declarer is by driving out their entries, before they are ready to use them.
Contract 3NT Lead ♥J
From the lead it is clear that declarer has the ♥K and ♥Q. So win the ♥A, and consider what declarer will try to do to make the contract. They will surely play on the club suit to promote winners there, so you should switch to a spade (your ♠K). This way you force declarer to win their ♠A, before they’ve had time to promote their club suit. Then, when declarer plays clubs, your partner will signal with the ♣2, showing an odd number of cards. This means that declarer has two clubs, so if you hold up your ♣A until the second round, you’ve broken declarer’s communications with dummy. Declarer will make seven tricks, but not nine, without the help of the club suit.
By switching to the ♠K, you took declarer’s entry to the club suit away before they wanted you to. The lead of the ♠K did cost a trick, because if you don’t play spades at all, you will make two spade tricks. But the sacrifice is worthwhile because you have stopped declarer making four clubs tricks.