As said before, when you coast, the motor generates electricity and sends it back to the car, the black thing is a capacitor that collects that power, stores it momentarily, and then sends it back to the motor when you hit the throttle. So it actually boost your performance when you coast through a corner then hammer it. You can go to Radio Shack and buy an electorlytic capactior that is larger than that one, and it will actually store more energy when your coasting and give a serious boost coming out of corners. Oh yea, and the part at the Shack cost like $.99!!! Check it out!

ps-if you cut it off, that electricity hits your fets, and speed control backwards, not good, save your electronics, don't cut it, upgrade it!