After the safety belt tension limiter accident, the safety belt was already strained under the pretension device. But we want to reduce the strain on the seat belt as soon as the stress peak passes, to reduce the stress on the occupant, and this particular task is done by the seat belt
The pre-tightening device described above wraps the safety belt underneath. Inside the core is a steel torsion bar. When the load reaches the predetermined condition, the torsion bar will start to twist, thus releasing the safety belt to some extent and realizing the tension limitation function of the safety belt.