FET's are the heart and guts of the speed control. FET stands for "field effect transistor." These transistors turn the power on and off to the motor. Thus making the vehicle accelerate more smoothly than the old 3-speed mechanical speed controls. They turn the power on and off at extremely fast speeds (depending on the Frequency the ESC operates at). The higher the freq. the smooth the throttle feels and the better run time you get. With the more accelerating and braking you do the more the FET's switch on and off, making the speed control run hotter. On long fast tracks heat sinks are not always needed, but on short tracks with many turns heat sinks act as insurance for your speed control.
