Ducking
This move consists of playing a low card from both hands, in order to facilitate communications between Dummy and Declarer.
Therefore you voluntarily give Opps a free trick.
There are many reasons why you should play this move :
- establishing a long suit
- an uneven suit distribution
- preparing a ruff
- preparing a squeeze
- ...etc...
Example : You're South playing a 3NT contract. You need to make 6
♣ tricks and you only have 2 entries to Dummy. You fear that one of the Opps has 4
♣ cards. What's your line of play ?