A trolley valve:

Question

The trailer hand valve (also referred to as the trolley valve or Johnson bar) is a mechanism that works the trailer brakes. It should only be used to test the brakes. Using it while the vehicle is being driven could cause a skid. Never use the trailer hand valve while parking because doing so may release the air pressure  from the braking system, releasing the brakes that are holding the vehicle in place.

Accepted Answer

Should only be used to test trailer brakes.

Answer

Should be used while driving.

Answer

Should be used when parking uphill.