When Not to Hold Up on Defence
A hold-up only works if it cuts declarer off. When dummy has another entry, or when you need the lead now, take your ace.
Holding up an ace against dummy's long suit can be a deadly defence. But it is not a rule to follow every time. A hold-up costs time, and sometimes time is what the defence is short of.
Ask two questions
Before ducking, ask:
Will ducking cut declarer off? Only if dummy has no other way in. If dummy has an outside entry, declarer can set the suit up whatever you do.
Do I need the lead now? If you can see the tricks to beat the contract, or a switch that must come before declarer's suit runs, win and get on with it.
Example: dummy has a side entry
You are East, defending 3NT. Partner led a heart and declarer won.
Dummy (North): ♠ 8 6 ♥ 9 5 ♦ K Q J 10 4 ♣ A 7 5 3
You (East) hold ♦ A 6 2 and ♠ K Q J 10.
Declarer leads a diamond. Ducking cannot strand the diamonds, because the ♣A gets declarer back to dummy. Every round you duck simply gives declarer time. Win the ♦A at once and switch to the ♠K. If partner holds the ♠A, the defence may take four spade tricks before declarer's diamonds come in.
Other times to win straight away
You can see enough tricks to beat the contract. Win and cash them.
Your ace is the only entry to your long suit. If you have set up winners in your own suit, you may need your ace to get in and cash them later. Ducking twice and then winning is fine, but do not let declarer take your entry away first.
Ducking gives declarer a trick. If declarer is short of tricks and your duck lets a low card win, you have handed over a trick for nothing.
Key takeaways
A hold-up is a way to cut communications. If it can't do that, think twice.
Count declarer's tricks: when time matters, win and attack.
Keep the ace you need as an entry to your own winners.