Defenders' Hold-Up Against No-Trump
When dummy has a long suit and no outside entry, holding up your ace until declarer has run out can cut the suit off completely.
Declarers learn to hold up an ace to cut the defenders' communications. Defenders can use exactly the same weapon against declarer. When dummy has a long suit and no other way in, your job is to win your ace at the moment declarer has no cards left in that suit.
A worked example
South opened 2NT and North raised to 3NT. Your partner (West) leads a spade.
Dummy (North): ♠ 7 4 ♥ 8 6 3 ♦ K Q J 10 5 ♣ 9 6 2 (6 HCP)
Declarer (South): ♠ A Q 5 ♥ A K 7 2 ♦ 7 4 ♣ A K 7 4 (20 HCP)
You (East) hold ♦ A 9 3.
Declarer has five top tricks outside diamonds: ♠A, ♥A K and ♣A K. With four diamond tricks that comes to nine.
Declarer leads the ♦7 towards dummy. Duck. Dummy's king wins. Declarer leads the ♦4 next. Now win your ace. Declarer has no diamonds left, dummy has no outside entry, and the ♦Q J 10 are stranded. Declarer makes one diamond trick instead of four.
If you win the first diamond, declarer leads the second diamond to dummy later and makes four diamond tricks, and the contract.
How long to hold up
Hold up until declarer has no cards left in the suit. To know when that is, count. Here you can see seven diamonds between dummy and your hand, so declarer and partner hold six between them. Partner's count signal on the first round tells you how they split: a high card shows an even number, a low card an odd number. Partner's ♦2 here (an odd number, three) means declarer started with two, so you win the second round.
Four common mistakes
Winning the ace at the first chance "in case it goes away". An ace against a suit with no side entry never goes away.
Holding up one round too many, so declarer's last card wins and the lead is in their hand anyway.
Forgetting to watch partner's count signal.
Holding up when dummy has an outside entry: then the hold-up gains nothing (see "When Not to Hold Up on Defence").
Key takeaways
Against dummy's long suit with no side entry, aim to win your ace when declarer has none left.
Count the suit and use partner's count signal.
Duck smoothly: a long pause tells declarer you hold the ace.