The trailer hand valve control (also known as a spike, a trolley valve, or a Johnson bar) controls the service brakes only on the trailer, not the tractor. You can use it to test the trailer service brakes independently of the tractor service brakes. Pressing the brake pedal activates the service brakes on the entire vehicle. The red dashboard knob controls the spring brakes on the trailer only. The yellow dashboard knob controls the spring brakes on the entire vehicle.