http://www.iihs.org/vehicle_ratings/...head_scion.htm


REAR CRASH PROTECTION When a vehicle is struck in the rear and driven forward, the vehicle seats accelerate occupants torsos forward. Unsupported, the occupants heads will lag behind the forward movement of their torsos. This differential motion causes the neck to bend back and stretch. The key to reducing whiplash injury risk is to keep the head and torso moving together. This is what a good seat/head restraint combination does.

Evaluations of rear crash protection are based on a two-step evaluation. In the first step restraint geometry is rated. Seats with good or acceptable geometric ratings then are subjected to a dynamic test. Seats with head restraints rated marginal or poor, based on geometry, aren't tested because they cannot protect taller people.

More about the procedures for rating seat/head restraints