What does the pinion snubber do ?

A rear-end does two things when power is applied. Due to engine rotation direction, the rear housing wants to rotate around the pinion gear, lifting the right rear tire, and planting the left rear. That's why an open rear will spin the right rear tire only unless something else changes the circumstances, such as a hard left turn, or a left donut.
The other thing a rear does, it tries to lift the nose (pinion) to rotate around the ring gear. The pinion snubber is supposed to control that.
I don't see how it can do anything. Most stock mopars, the snubber it a few inches away from the floor, so it doesn't bottom out the suspension over every little bump. So, it can't do anything.
So they make adjustable snubbers, to get them up against the floor, for a hard launch. Only, in my experience, most mopars LIFT the body to plant the tires at launch, and the snubber is waving in the air again.
My adjustable snubber is holding down papers in my garage, and my car is a few pounds lighter.