If the rear seat backrests are not locked, they could fold forwards in the event of an accident, heavy braking or sudden changes of direction.
As a result, child restraint systems cannot perform their intended protective function. Rear seat backrests that are not locked can also cause additional injuries, e.g. in the event of an accident.
If the rear seat backrest is not engaged and locked in place, the red lock verification indicator will be visible.
If the child restraint system is equipped with a Top Tether belt:
The risk of injury can be reduced by a Top Tether. The Top Tether belt facilitates an additional connection between the child restraint system attached with ISOFIX or i‑Size and the vehicle.
Vehicles with adjustable head restraints:
under the head restraint between the two head restraint bars. Vehicles without adjustable head restraints:
over the centre of the head restraint. past the head restraint on the right and left side respectively. All vehicles:
into Top Tether anchorage 
without twisting. . Comply with the child restraint system manufacturer's installation instructions when doing so. Vehicles with adjustable head restraints:
.