2012 Toyota Prius Owners Manual - Page 399

1-4. Adjustable components (seats, mirrors, steering wheel) Seat belt pretensioners (front seats) The pretensioner helps the seat belt to quickly restrain the occupant by retracting the seat belt when the vehicle is subjected to certain types of severe frontal or side collision. The pretensioner does not activate in the event of a minor frontal impact, a minor side impact, a rear impact or a vehicle rollover.