If you are talking about the idle-down function found in Futaba (and probably others) radios, it is a function that lets you choose two idle rpm settings. I use this function with my Extra 300. High idle for aerobatics, to be absolutely sure that the engine won't quit when I chop the throttle, regardless of attitude, and when it's time to land I flick the idle-down switch to get lower idle and minimum thrust. It is basically two different idle rpm settings for different "flight modes".