Some vehicles are equipped with a different type of cab-mounted trailer-supply valve, which must be operated manually by the driver. It has two positions: NORMAL and EMERGENCY. The important difference is that this trailer-supply valve must be shifted to the EMERGENCY position manually.
Charging the trailer system:
The driver places the trailer-supply valve in the NORMAL position and reservoir air will be directed to the tractor protection valve and supply-line glad hand.
Rapid loss of air pressure in the supply line will cause the trailer brakes to dynamite. “Dynamiting” is an emergency application of the trailer brakes.