The best I can figure,theyre securing them back down to the floor.
I even saw a neighborhood youth program car at a car show here in Austin, built by the local police youth tack force, with just such a set up.
At which point I asked them if they were going to make the set up safe? They looked at me like a calf at a new gate!
It is possible to secure the outside sholder belts to the normal mounting point above the B pillar, but you either need a horizontal roll bar or some sort of a central tower to secure the other side to. Not doing this and having an accident with such a set up risks permenant parrallisis from spinal compression. Please be safe.
Andrew