Why should the trailer hand valve not be used while the vehicle is in motion?

Using the trailer hand valve while the vehicle is in motion can cause the trailer to skid due to the sudden application of brakes on the trailer wheels. This is particularly concerning because the braking force applied to the trailer can differ from that of the tractor, leading to instability. When the trailer brakes are engaged abruptly while the vehicle is moving, the rear of the truck could slide sideways, creating a dangerous situation that could result in loss of control of the entire vehicle. Proper braking techniques and understanding the physics of vehicle dynamics are crucial for safe vehicle operation, especially with a combination of vehicles like a tractor-trailer.

The other choices relate to different concerns: disengaging the brakes might imply a malfunction, losing air pressure would be a serious safety issue but is not directly tied to the function of the hand valve while moving, and reduced engine power does not pertain to the operation of the trailer brake system.