A pretensioner is designed to retract some of the webbing of a seatbelt the instant a collision occurs, tightening the seatbelt to restrain occupants quickly and reducing the amount they are thrown forward in a moderate or severe frontal crash.
Load limiters help protect occupants from seatbelt-inflicted injury. In the event of a crash, the pretensioner restrains the occupant until certain amount of force is applied. At this point the load limiter releases the webbing gradually so as not to exert too much force on the chest of the occupant.
How Seat belt with pretensioner and Load limiter works