Answer :
Final answer:
A driver uses the trailer hand control valve to operate the trailer service brakes independently of the tractor service brakes.
Explanation:
To apply the trailer service brakes independently of the tractor service brakes, a driver uses the trailer hand control valve. Unlike the one-way control systems described, where a brake system can prevent movement in all directions and a valve system can control the flow of liquid in one direction while allowing it in another, the trailer hand control is designed specifically for the modular operation of a commercial vehicle's brakes. This is particularly useful for maneuvering at low speeds or when coupling and uncoupling trailers.
