Seat Belt PRETENSIONERS (Emergency TENSIONING Retractor)
WARNING: This page is about a different car, the 2001 BMW X5. However, it is still accessible from the selected car via links, so may be relevant.
Seat belt retractor or Emergency Tensioning Retractor (ETR) is located in seat belt buckle. When front air bags deploy, air bag control unit also deploys seat belt retractor which increases seat belt tension and holds front seat occupant(s) tightly against seat backrest, increasing seat belt effectiveness.