On a combination vehicle, what is the purpose of the trailer air supply valve (in the cab)?

The TRAILER AIR SUPPLY VALVE (a control in the cab, often a red octagonal knob) controls air to the trailer. CDL combination. PUSHED IN: supplies air to the trailer, charging the trailer air system and RELEASING the trailer's spring (emergency) brakes — this must be done before driving with a trailer. PULLED OUT: cuts off air to the trailer and APPLIES the trailer's spring (emergency) brakes (used for parking the trailer or in an emergency). The valve also pops out automatically if trailer air pressure drops too low (e.g., a broken air line), applying the trailer brakes. Before driving, the driver pushes it in to charge the trailer; the system should hold pressure. Knowing the trailer air supply valve's function (push in to supply air/release trailer brakes) is commonly tested.