<P> The purpose of locking retractors is to provide the seated occupant the convenience of some free movement of the upper torso within the compartment, while providing a method of limiting this movement in the event of a crash . Most modern seat belts are stowed on spring - loaded reels called "retractors" equipped with inertial locking mechanisms that stop the belt from extending off the reel during severe deceleration . There are two main types of inertial seat belt lock . A webbing - sensitive lock is based on a centrifugal clutch activated by rapid acceleration of the strap (webbing) from the reel . The belt can be pulled from the reel only slowly and gradually, as when the occupant extends the belt to fasten it . A sudden rapid pull of the belt--as in a sudden braking or collision event--causes the reel to lock, restraining the occupant in position . </P> <P> A vehicle - sensitive lock is based on a pendulum swung away from its plumb position by rapid deceleration or rollover of the vehicle . In the absence of rapid deceleration or rollover, the reel is unlocked and the belt strap may be pulled from the reel against the spring tension of the reel . The vehicle occupant can move around with relative freedom while the spring tension of the reel keeps the belt taut against the occupant . When the pendulum swings away from its normal plumb position due to sudden deceleration or rollover, a pawl is engaged, the reel locks and the strap restrains the belted occupant in position . Dual - sensing locking retractors use both vehicle G - loading and webbing payout rate to initiate the locking mechanism . </P> <P> Seatbelts in many newer vehicles are also equipped with "pretensioners" or "web clamps", or both . </P> <P> Pretensioners preemptively tighten the belt to prevent the occupant from jerking forward in a crash . Mercedes - Benz first introduced pretensioners on the 1981 S - Class . In the event of a crash, a pretensioner will tighten the belt almost instantaneously . This reduces the motion of the occupant in a violent crash . Like airbags, pretensioners are triggered by sensors in the car's body, and many pretensioners have used explosively expanding gas to drive a piston that retracts the belt . Pretensioners also lower the risk of "submarining", which occurs when a passenger slides forward under a loosely fitted seat belt . </P>

Who is responsible for wearing a seatbelt in a car