"Cocoon" is an unfortunate choice of words as it implies some increased protection. Rear-facing seats are not designed to do this for any safety purpose, that's just how they perform in a crash and someone attached that name to it.
Anti-rebound features (tether, bar, foot, etc) on a rear-facing seat will reduce head excursion and help keep the child from striking part of the interior on a rebound (or in a rear impact and rollover). It is the same purpose that a standard top tether serves on a front facing seat.
The difference is that rear-facing is so safe, the addition of the tether is not nearly as important of an improvement as it is for a front facing seat. The energy in a rebound is less than the initial forward energy in a frontal crash. That is one reason why most manufacturers opt to save the costs for additional design and testing and do not allow rear-facing tethers.
The important thing is to keep your child rear-facing to at least 20 pounds AND 1 year at an absolute minimum, longer if at all possible. Keep them correctly restrained in the back seat and make sure the child seat is installed correctly. If you've done that, your child will be very safe. An anti-rebound feature in a rear-facing seat is icing on the cake; nice to have, but not nearly as important as these other factors.