sorry guys i should have explained myself better, i think it's a fail safe, so if the car goes out of range it will apply full brake and not full throttle
Plug it into the throttle channel, then plug the throttle servo into the other end. Program it to turn down the throttle and apply some brake.
Test it by turning off the Radio while the receiver is still on. No need to run the engine during the test. Try it a few times just to make sure you've got it programmed where you want it.
most of the ones I've seen, you hold the trigger where you want it, (slight brake) and press the button on the failsafe. that should lock the position in. as rolex said, turn off the transmitter and see if it goes to your pre defined position.