Why is it dangerous to use the trailer hand valve while driving?

Using the trailer hand valve while driving is dangerous primarily because it may cause the trailer to skid. The hand valve is designed to apply the trailer brakes independently of the vehicle's service brakes, and if engaged while the vehicle is in motion, especially in a turn or at high speed, it can lead to uneven braking between the tractor and trailer. If the trailer brakes are applied while the truck is moving, it can create a loss of traction at the trailer wheels, potentially causing the trailer to skid. This uneven braking can also lead to a loss of control of the entire vehicle combination.

To understand the other potential consequences, while the trailer could detach if there are issues with the connections or if it were severely out of control, this is not a direct result of using the hand valve. Reduced braking efficiency is a concern, but it is more about how the hand valve engages and not like it would be when braking normally. Complete brake failure is a more extreme scenario that does not directly relate to the act of using the hand valve while driving. Therefore, the principal danger is indeed the risk of skidding, which can lead to loss of control and potential accidents.