AMD's have Cool n Quiet which allows them to underclock when they aren't being used as much to keep things cooler and possibly slow the fans down. I beleive all(desktop and mobile) versions do similar things. I know it's quiet obvious on my laptop with it's Sempron. I've never seen it down to 400MHz! But it is quite commonly around 800MHz... I don't know if it just halfs the speed, or if the Athlons do any more...