Properly positioned lap belts control both vertical and horizontal
motion. The shoulder belts keep the torso from pivoting forward and
the crotch strap keeps the lap belts in place. Note that the crotch
strap need not actually contact your crotch to properly perform this
function. My recollection of the racecar research is that, in a
reclined seating position, a substantial portion of the force on the
shoulder belts is transfered downward and captured by the lap belts.

Ray Warshaw