Tether strap for RideSafer vests. Optional if you're using a 3-point seat belt, recommended for wriggly kids. If you're using a lap-only seat belt, your vehicle must have a top tether anchor  in that seating position and you must use the RideSafer in conjunction with the tether strap. 

The manufacturer recommends the additional RideSafer neck pillow accessory (available separately) when the vest is used with the tether so that children with longer hair will not have their hair caught in the tether rings.