Generally the seat and shoulder belts are loose so that the drive and passenger can have the freedom to move forward to adjusting the instruments on the dash , etc. However, when the vehicle decelerates quickly, as in a crash or panic stop situation, the occupants need to be restrained by the belts. The belts are locked by the inertia reel as it senses the rapid change of deceleration .
INERTIA REEL
Meaning of INERTIA REEL in English
English dictionary of automotive terms. Английский словарь автомобильных терминов. 2012