What can a driver use the trailer control valve (hand valve) for?

The trailer control valve, commonly referred to as the hand valve, is specifically designed to allow the driver to apply the trailer’s service brakes independently of the tractor's brakes. This feature is particularly useful in situations where a more gradual or controlled braking effect is needed on the trailer, especially in conditions such as backing up or when maneuvering in tight spaces. By using the hand valve, the driver can exert more force on the trailer brakes relative to the tractor brakes, which can enhance control and stability.

This function is critical for maintaining safety and ensuring that the vehicle operates smoothly under various driving conditions. While the other options mention aspects related to braking systems and functionality, they do not accurately capture the primary purpose of the trailer control valve. The hand valve is not utilized for releasing parking brakes, illuminating brake lights, or adjusting air pressure—it solely focuses on enhancing trailer braking.