Inertia keeps the occupant's head from moving, so it moves backward in relation to the rest of the body and the car.
Occupant rebound occurs, where the head finally speeds up and catches up with the rest of the body.
Flexion occurs in this phase, and if the collision is severe enough, the occupant may strike her head on the steering wheel. This theory worked fine until engineers found that occupants were reporting injury in many low to moderate speed collisions. This was confusing, because the neck did not hyperextend at these low speeds, but was well within the range of normal spinal motion. In short, the basic model of whiplash mechanics was inadequate. The answer lies in the complex anatomy of the neck.