"Seatbelt pre-tensioners are used to tighten the seatbelt in a severe frontal accident. Both mechanical and electronic control systems are available. The most common type relies upon an explosive charge that is detonated electronically by a sensor within the seatbelt tensioning mechanism.

This explosion moves a piston that pulls on a steel cable causing the belt to tighten by approximately 4 inches or 100 millimeters. The design allows for the belt to tension before the occupant has moved forward in the seat.

Mechanical systems rely on inertia to move a sensing mass. This releases a spring to pull on a cable, thus tightening the belt.

Once the pre-tensioner has triggered, a ratchet prevents the seatbelt from loosening. When the seat belt is removed from the buckle, it cannot be reinserted, and the assembly should be replaced.

Rip stitching is used on seat belts in conjunction with an air bag and seat belt pre-tensioners. During a collision the pre-tensioners initially pull the seat belt tight, however the stitching gradually tears to allow the occupant to move forward into the air bag at a controlled rate.

For safety reasons, these belts must be replaced once they have had their stitching ripped. Manufacturers generally fit warning labels within the fold to indicate the belt is to be replaced when the label is revealed. "

http://www.cdxetextbook.com/auxil/srssyste...etensioner.html

 

Most Hyundai seat belts have a ratcheting mechanism that stops working after an accident. The driver's side belt in my Elantra would no longer retract after I got hit.