In normal driving conditions, what is a reason for using the hand valve before applying the brake pedal?

Using the hand valve before applying the brake pedal is essential in certain driving situations, especially when towing a trailer. The primary reason for this is to ensure that the trailer begins to brake before the tractor, which is crucial in preventing a jackknife situation. A jackknife occurs when the trailer swings out to the side of the tractor, which can lead to a loss of control and potentially a serious accident.

By applying the hand valve, the driver engages the trailer brakes first, allowing for a more controlled and gradual deceleration. This sequence of braking helps maintain the alignment between the tractor and trailer, reducing the risk of the trailer overriding the tractor's braking action and causing a dangerous angle or jackknife.

While smoother braking is beneficial, the specific purpose of the hand valve in this context is primarily to maintain safe control while towing, making it a critical operational tool for drivers of combination vehicles. Each of the other options does not accurately relate to the functional use of the hand valve in this situation, underscoring its specific role in enhancing driving safety.