A shoulder harness is designed to hold you back, not down.

When most racecars are built, the chassis builder puts the bar behind the seats in the best position for the chassis, which is not necessarily in the best position to mount your shoulder harness. When the chassis harness bar isn’t in the right position, mounting the shoulder harness at the correct height can be a complicated task. In most cases, the shoulder harness ends up being mounted below the shoulders which puts a vertical load on your spine when tightened. Mounting the shoulder harness below shoulder height is not only uncomfortable, it’s dangerous. In a situation where the harness is too low, the harness ends up in an arc over your shoulders which allows front to back and side to side movement. The movement not only increases fatigue as you try to hold yourself into the seat, it doesn't allow the harness to do its job and gradually slow your upper torso down as it stretches in the case of an impact. In an impact with a loose harness, your upper torso slams your torso against the harness before it starts to stretch, which often times will cause neck and back injuries.
There's a saying, "If you don't swing the hammer, you can’t drive the nail". You’re the hammer when you don't have a properly adjusted shoulder harness.

The Shoulder Harness Adjuster is also designed to pull the D-Ring back from the seat to create more space and reduce binding when the seat is backed up to the harness bar.

The shoulder harnesses are held with 3/8 Grade 8 bolt through a 5/8 120 Wall DOM sleeve which surpasses the strength of the 7/16 hardware commonly required for harness mounting.