It has a little electric "servo" motor inside it that drives some clockwork gears that do 3 things. It drives a vacuum switch that changes which vents have air coming out via vacuum pods that actuate flaps in the dash. It also controls the blower speeds and opens/closes a water valve at the base.
There are some pics of mine disassembled here:
http://www.peachparts.com/shopforum/diesel-discussion/304961-its-heater-season-acc-fun-2.html