I've never owned a bike with any kind of suspension or damping beyond fatter lower pressure tires, so I'm really out of the loop here. I can't understand how this would work when the chainstays are solidly welded or brazed to the bottom bracket shell. Does this make sense? I always assumed you would need some type of paralleolgram movement for the rear triangle, a lower pivot of some sort.

I guess I just missed out on a couple decades of technology. Seems like a lot of hardware for a couple mm of potential frame travel. What am I missing here? Is this common on some mountain bikes?