Here are three very different features, yet all do the same thing – they keep the rear facing child’s head and neck safer by preventing the rear facing car seat from reclining any further during a crash.
The more upright the rear facing car seat stays during a crash, the more the child’s head, neck and back get cradled by the shell, allowing the shell to absorb more of the crash forces, putting less force into the child’s body.
The more reclined the rear facing car seat becomes in a crash, the more the child slides up the car seat, which loads the shoulders and neck with more of the force. Also, the more the child slides up the seat the higher the risk of their head hitting the back of the front seat.