Which component is utilized to apply the brakes on a trailer during testing?

The hand control, often referred to as the trolley valve, is an essential component specifically designed for applying the brakes on a trailer during testing. This control allows the driver to manually engage the trailer's brakes independently of the standard foot brake used for the vehicle. By pulling the hand control, the driver can test the responsiveness and functionality of the trailer's braking system, ensuring that it operates correctly without relying solely on the driver's foot brake.

This is particularly important when performing safety checks before a trip and when checking the braking performance of the trailer under various load conditions. It helps ensure that the trailer's brakes are capable of stopping effectively when required, contributing to overall safety on the road.