It's a small device that plugs into the reciever where the throttle/brake servo usually plugs in. Then you plug the throttle/brake servo into the failsafe. With the truck power switch on, you hold full brake (or desired brake position) on the transmitter and push the button on the failsafe. The failsafe is now programmed for that predetermined brake position. That's all you have to do.
Now anytime you lose radio signal or battery power, the failsafe will move the throttle/brake servo to your programmed brake position automatically, cutting the throttle, and preventing the truck from running away out of control.