"In a hot-air system, warm air is distributed via a main duct or
"trunk" and a series of branches that lead to individual rooms or
zones. Where the branches meet the trunk, heat is controlled by dampers
which open or close to release or block heat from entry. These dampers
are usually motorized and run by thermostatic controls at each zone.
Individual registers may also be closed to block heat, but this is a
less efficient use of the energy and heat produced than thermostatic or
automatic controls."
is says at:
http://www.bobvila.com/ArticleLibrar...ForcedAir.html

pictures of dampers at:
http://www.zoneatrol.com/products.html