The Simpson & Hans devices are made to prevent the head from going forward during impact (Earnhardt)which is the issue in an auto frontal type crash since the body is restrained by the seat belts. . . .
I agree. The primary reason for a HANS device is to prevent a basal skull fracture, where the skull snaps at the point it connects to the spine. This caused by hyperextension, which in turn occurs because the seat belts stop the torso from moving, but the head/skull in not similarly restrained and thus continues to move forward. The HANS device ties everything together so that the entire upper body now is restrained.