What occurs to the trailer during a brake application controlled by the relay emergency valve?

During a brake application controlled by the relay emergency valve, air is relayed to the trailer brake chambers. This is crucial because the relay emergency valve is designed to ensure that the trailer brakes respond promptly and effectively in conjunction with the tractor's brakes. When the driver applies the brakes, the valve facilitates the transmission of air pressure from the tractor to the trailer's braking system with minimal delay.

This functionality helps maintain stability and control while braking, especially in heavier vehicles where the trailer may lag in braking response if the system relied solely on the tractor's brake application. Effective relay of air pressure is vital for the safe operation of combinations of vehicles, ensuring that they stop in a synchronized manner to prevent jackknifing or loss of control.

In contrast, the other options do not accurately describe what happens in this scenario, as the mechanism is specifically intended to assist in braking efficiency and coordination between the tractor and trailer.