Britax Safe'n'Sound Instructions For Installation & Use page 26

HOW TO INSTALL - TOP TETHER STRAP
IMPORTANT :
The Top tether strap must be fitted at all times when the baby is in the car.
Always secure the loose end of the top tether strap to prevent it hanging out of the
car or being accessible by the child.
THIS RESTRAINT IS NOT SUITABLE FOR USE WHERE THE TOP TETHER
STRAP MAY FALL INTO A SPLIT IN THE SEAT BACK OR OFF THE SIDE OF
THE SEAT.
ALWAYS ATTACH TOP TETHER HOOK AND REMOVE SLACK IN THE TOP
TETHER STRAP.
CAUTION:
CHILDREN CAN BE INJURED OR RESTRAINTS AND TOP TETHER STRAP
DAMAGED BY UNSECURED CARGO IN A COLLISION. SECURE OR REMOVE
HAZARDOUS CARGO BEFORE USING THIS CHILD RESTRAINT.
REMOVING THE BASE FROM THE CAR
When removing the base, release the attachment clip from the car's upper anchor fitting
then remove the vehicle seatbelt.
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Instructions for Installation & Use