What is the primary use of the hand valve when towing a trailer?

The primary use of the hand valve when towing a trailer is to test the trailer brakes. When using the hand valve, a driver can apply the brakes on the trailer independently of the towing vehicle. This function allows the driver to check if the trailer's braking system is working correctly and whether it is properly synchronized with the tow vehicle. It is crucial for safety to ensure that both the trailer and the vehicle can stop effectively during operation.

While the other options involve various functionalities associated with towing and vehicle operations, they do not pertain to the specific function of the hand valve. Testing the brakes is an essential practice for ensuring safe operation, whereas adjusting air pressure, activating turn signals, and engaging the throttle involve different systems and controls within the vehicle or trailer mechanics.