When your belts are wrapped around the harness bar, they have a tendency to slide all over. PRP belt minders keep both shoulder belts together on the bar. Stops them from sliding apart, and is much better for safety. The belt minder should be located behind the back of the seat and the harness bar.
Easiest to install by unwrapping each shoulder belt from the bar, running it through the minder, and then reattaching the shoulder belt to the bar.