The Movcam Rosette-Articulating Shoulder Pad Assembly with 15mm Clamps mounts 15mm rods at the front, rests on your shoulder at the back, and articulates in between by a locking rosette mechanism.
At the back, right atop the shoulder pad, is a 15mm rod clamp like the one in front but upside down. It lets you attach 15mm rods to the back to support accessories like power adapters, batteries, and recorders. These accessories will also happen to provide counterbalance to the rig in front.
Non-slip rubber shoulder pad