Let's take a rear end collision.  As the head travels backward, it can either hit against a good headrest, or if the headrest is too low or not there, it travels backward until the facet joints in the back of the spine and/or the ligaments in the front of the neck stop it. The most commonly injured muscles are the SCM's and longus colli.

The most recent research suggests that rather than coming straight back, the neck actually takes on an s-shaped curve in a rear end accident.  The medical and scientific research is only now starting to show us the mechanisms behind the injuries seen in car accidents.   Many car makers are now designing seats that cam absorb the forces on the head in a rear end impact.

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