When your vehicle is hit from the rear, the impact causes the head and neck to first be forced backward as the seat pushes your torso forward. This is followed by an opposite movement when the head and neck are thrown forward rapidly.
These rapid back and forth movements cause the cervical spine to contort into an unnatural S-shape. The soft tissues that hold your spine together (ligaments, facet capsules, muscles) can become hyper-extended, leading to small or large tears and compressions in the tissues.