The **headrest** is a passive safety device designed to protect the neck and cervical spine in the event of a sudden impact. Its correct adjustment is crucial to prevent injuries such as **whiplash**, which occurs when the head moves violently backward and then forward, typically in rear-end collisions.

- For the headrest to be effective, it must be at the same height as the top of the passenger's head. Additionally, the distance between the head and the headrest should be four centimeters or less. This ensures that the headrest can adequately stop the head's movement during an impact.

- Unlike whiplash, the **submarining effect** and **driver fatigue** are not directly related to the adjustment of the headrest. The submarining effect is caused by improper seatbelt placement and incorrect posture, while fatigue is related to poor driving posture.

Therefore, correctly adjusting the headrest is essential to reduce the risk of suffering whiplash in the event of an accident.