What is the function of the seat belt pretensioner in a vehicle's Supplementary Restraint System (SRS)?

1. To adjust the height of the seat belt anchor based on passenger height
2. To quickly tighten the seat belt during a collision to secure the occupant in their seat
3. To release seat belt tension after a crash so occupants can exit the vehicle
4. To chime a warning if the passenger does not buckle their seat belt