be80be:
It's a dimmer http://www.lutron.com/en-US/Products/Pages/StandAloneControls/Dimmers-Switches/RotaryDimmer/Overview.aspx
http://digital.turn-page.com/t/23303/119 1500 watt triac dimmer
Oh, I see. Well most likely the dimmer is actually a rheostat anyway - at least I imagine this to be the case. By turning the nob you are adjusting the resistance and limiting current to the light. Less current through the bulb filament means less heat and therefore less light.
In this application you are limiting the current to the iron and effectively lowering its power consumption (by less current) thereby making its operating temperature lower.
Thanks for the link. I think I will try this myself.