Originally Posted by Juha_Koo
AFAIK it's a safety device designed for karting use. In karts seat's rear side is very short and therefore rear impact is really dangerous because the upper body (including the neck) could whipsplash rearwards.
In rallying that device would help if used the wrong way around (the bulky side in front) to stop head from whipsplashing forward (because seats are high enough to provide head support backwards). In the past, I've seen it being used like that in Finland, but it ended when ASN doctor warned competitors that it could lead to severe cervical spine and throat injuries as it has not been desinged to be used that way around...