The MPRS 80mm Multi-Purpose Sliding Rail with L-Plate from Really Right Stuff is designed for the Canon 800mm and 600mm f/11 prime lenses. These lightweight, extendable lenses makes proper balancing difficult; the MPRS-80 solves that problem by allowing the foot to slide back to the balance point when the lenses are extended for use, and forward when in the stowed position. Since the lenses lack a collar, holes are provided for mounting the L-plate to the foot, allowing quick and easy vertical orientation.
The MPRS-80 is best for back-heavy setups such as when using battery grips. (The optional MPRS-40 is best for lightweight setups.)