a harness that does not extend horizontally back from your shoulders will compact your spine in an impact. harness that are bolted to the floor or lower rear seat belt positions are examples of harness that extend below horizontal. in the event of an impact, as the human body pulls on the non horizontal harness reponds with forces in the x and y directions to restrain the occupant. this is simple physics. the magnitude of the the y direction Force (up /down) is proportional to the angle below horizontal of the rearward straps. this means the greater the downward angle of the straps the more spine compression you are going to get.

ALSO! DO NOT run a harness without a cage or roll bar. in the event of an overturned vehicle the roof will collapse in on the occupant. a normal setbelt is designed to allow the occupants to shift side to side to avoid the collaped roof. in a harness you can NOT move side to side and avoid this. you will break your neck as a result if the roof comes in far enough. the only way to prevent this is the installation of a cage or roll bar that prevents the collapse of the roof.