Seat Belt Pretensioner(hereinafter referred to BPT) is located at both side of center pillar.

BPT tightens seat belt before air bag deploys to protect passenger from bumping against crush pad, steering wheel and front window.

In BPT, there are a ignition circuit and cylinder for rewinding belt.

Cylinder has piston that can rewind seat belt in it.

Gas chamber generates expansive force of gases to push piston in cylinder.