Question about Britax Römer Kidfix XP SICT

The base of the seat is held in place by the ISOFIX system. The back of the seat, which is rather light, seems to adjust its angle to that of the car seating it is attached to. The child is actually held in place by the normal seat belt, which passes through a sleeve with padding. The main purpose of the back would seem to be the protection for the child's head and torso from side impacts. It also holds the child in place much better than would be the case only with the seat belt.
I am not an expert on children's car seats and we purchased this seat for my wife's car as our daughter had already chosen the same model. Various reviews gave the seat a good score (4.5/5.0) and as a physicist I haven't found any obvious flaws - assuming the child has reached the recommended height.
I hope this helps...