Seat Belt PRETENSIONERS
WARNING: This page is about a different car, the 2001 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.
The seat belt pretensioner is a pyrotechnic device, which is detonated by the air bag control module. This reaction produces a quantity of gas that pushes a piston down inside an aluminum tube. The piston pulls a cable attached to seat belt roller and tightens the belt.