The trailer hand valve (also known as a spike, a trolley valve or a Johnson bar) controls only the trailer brakes, not the tractor brakes. You can use it to test the trailer brakes. But don’t use it as a parking brake. (Use the real parking brake control, which will set the brakes on both the tractor and the trailer.) Don’t use the trailer hand valve while you’re driving either. Braking the trailer but not the tractor could cause a skid.