Tesla S Owner's Manual page 21

Seat Belts
Warning: Never place anything between
you and the seat belt to cushion the
impact in the event of an accident.
20
Seat Belt Pre-tensioners
The front seat belts are equipped with
pre‑tensioners that work in conjunction with
the airbags in a severe frontal collision. The
pre‑tensioners automatically retract both the
seat belt anchor and the seat belt webbing,
reducing slack in both the lap and diagonal
portions of the belts, resulting in reduced
forward movement of the occupant.
If the pre-tensioners and airbags did not
activate in an impact, this does not mean they
malfunctioned. It usually means that the
strength or type of force needed to activate
them was not present.
Warning: Once the seat belt pre-
tensioners have been activated, they must
be replaced. After any accident, have the
airbags, seat belt pre-tensioners and any
associated components checked and, if
necessary, replaced.
Model S Owner's Manual