second—Sensors detect the impact
As the satellite sensors (collision sensors) attached to the vehicle detects the collision, a signal is sent to the ECU (Electronic Control Unit). When the speed of the vehicle suddenly drops because of impact, a separate vehicle sensitive locking feature located in the retractor detects the negative acceleration (rate of deceleration) and locks the webbing to keep it from further extraction. This restrains the forward movement of passenger. (The vehicle sensitive locking feature also goes into operation when the driver applies the brakes suddenly or turns the steering wheel sharply.)
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Key Point!
When you pull the seat belt webbing suddenly from the housing during normal use, it also locks. However, this is due to the operation of a separate webbing sensitive locking feature. Retractors are equipped with both vehicle sensitive and webbing sensitive locking features to further enhance safety.