Unless it is mechanically attached to the rear seat frame, it won’t provide protection in a crash. Too many dynamic forces involved for a slip on headrest to function safely. Screw it to the frame of the backrest and then maybe.
No such thing as a perfect direct from behind impact in a straight line...and that thing could do more harm that good being a slip on. A few degrees off from center and your back or neck is fractured by it.
