If you look underneath the rear of the car, the suspension arms are mounted to something that is itself mounted to the body of the car (the "subframe"). There are rubber/soft mounts that i assume minimize vibrations travelling into the car body. You can also see in that subframe two holes where the subframe lock bolts would go. Look through the holes to see the threaded holes in the body, that's where the bolts go.
Seems to me this is similar to stiffer bushings, stiffer sway bar mounts, top mounts, etc. The goal being to reduce suspension flex.