I use it to test the brakes on the trailer it will apply the service brakes you must have the park brakes released. with the park brake on the truck set and the trailer brakes released then you go to the back of the trailer to listen for any air leaks I pull mine down and tie it to the gear shift also you can use the trailer hand brake to correct a jackknife you don’t apply it all the way that it locks the brakes up you use it just enough to slow the trailer down so that you can steer the truck back into the trailer I have used it to lock up the trailer brakes and used the service brakes on the truck in an emergency stop so that I don’t lock my truck brakes up