Programmable 'burst' control thingy. Has a low battery warning which is handy and programmable burst from 1 to 12. Got it set at 5 at the mo but only takes seconds to change. Installed it in my XR5 which has a HUGE battery compartment, I run it with a 4400 mah lipo, and it all tucks in nicely. First impressions are very good. Will try it next time out and see how it does.