Ducking can be used for multiple purposes (not necessarily all on same hand, of course).
Some uses of ducking
- Entry creation: This IMO is the primary use. Sometimes you duck in your side suit to make sure you have enough entries to establish the suit and cash it. Notice that the primary use of hold-up is to prevent opponents from having entries to cash a suit.
You are in 3NT, playing rubber bridge. Opponents lead a spade.
You have 5 tricks outside of diamonds and need at least 4 from the diamond suit. You have to hope diamonds break 3-2.
If you play diamonds from the top, on a 3-2 break, you will lose the third round, and establish the suit, but you will not have a way to cash them.
But if you duck the first round of the suit, then you have lost a trick while you hold a small diamond in your hand. Now once you get back in, you can play a diamond to dummy and play AK drawing the opponents diamonds and are in dummy to cash the rest of the suit.
- Avoidance: You duck a suit making sure an opponent who cannot harm you will win the trick.
You are play rubber bridge and have gotten to a diamond small slam.
LHO leads the spade 7.
You have 11 top tricks, and the 12th can either come from either the spade finesse, or by establishing a long heart.
If you play the SQ at trick 1, RHO might with the K and knock out your spade A, thus removing your option of establishing the long heart too.
The correct play is a low spade from trick 1. The 9 in your hand ensure that your RHO has to win it, who cannot attack spades without giving your the 12 trick.
This play also lets you combine your chances.
Say RHO plays back a club. Now you can draw trumps, play AK of hearts and ruff a heart.
If hearts prove to be 4-1, you can fall back on the spade finesse.
- Timing: You might duck to rectify the count of a squeeze, at a time it is convenient to you, etc.
A common example (for defence) which is cited for this is the following:
South is playing 6NT.
Say West leads a spade and East should hold up. Declarer wins and play a spade. East should hold up again. Now if declarer plays another spade, East can cash two tricks. If declarer abandons spades, no matter what he does he cannot avoid losing two tricks.
If east takes either the first or second spade, declarer's count is rectified and West gets squeezed in the minors.
As with almost every play, there are times where it would be a bad play. For instance, you duck and realize you just gave away the setting trick :-)