inertia reel 
noun
A reel device which allows a vehicle seat belt to unwind freely but which locks under force of impact or rapid deceleration.It was a modern inertia reel type of seat belt system....
- Six airbags, five 3-point inertia reel seatbelts, an energy absorbing side impact protection system, a ring of steel surrounding the car and high strength steel beams at front and rear are standard.
- With the inertia reel belts used in cars nowadays, it is vital to ensure the belt is pulled very, very tightly around the seat to ensure it stays tight and the seat cannot wobble.