Seat Belt PRETENSIONERS (Emergency TENSIONING Retractor)
WARNING: This page does not describe the selected car, but rather 10 other vehicles, including the 2001 BMW Z3, 2001 BMW M3, 2001 BMW M Roadster, 2001 BMW M Coupe, and 2001 BMW 330xi. 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.