In the days before retractable seat belts, the non retractable ones would flop all over the place when not being worn & became a pain.

These were an aftermarket accessory that were pre wound, then slipped onto a seat belt. When you let the belt go, they would wind up & shorten the belt, pulling it out of the way.

It was a requirement that the belt be not wound up in any way when it was being worn & some lazy people that didn't adjust the belt properly found themselves in serious pain if there was still 4" of belt wound up when they needed the belt the most.