Control bidding is done with strong hands to examine the feasibility of a slam contract.

When it is your turn, while bidding controls, you call the cheapest suit in which you possess a not yet mentioned control. Aces and Kings are always controls. When a trump suit has been established voids and singletons are also controls

The exchanging of controls is started when:

The control bidding or exchange of controls is at least continued until the game level is reached.
Normally the initiator (the player, who gives the initial sign and who often has hidden strength and/or most information) of the control bidding decides about: If you want know more about slam bidding (also for grand slam) you should read the Blackwood page, where a suggestion is done about additional agreements that can be in this respect.
In that chapter you may also find a complete list of situations, where a trumpsuit is estabished.

When you want to see bridge games, in which the principles, described above, are applied, you should click for examples, and make your choice when you're arrived there.