The other night my dad had to brake hard in his 2004 Mercedes S320cdi, as he did both our seat belts tightened up to hold us into the seat, once he had stopped, and went to drive away the belts loosened and all was fine again. Is this a belt pretensioner? Because, I always thought a belt pretensioner was the device that stopped it snatching forward if you pull on it hard...

 

Thanks, Dan

These days seat belt mechanisms are quite intelligent. The inertia mechanism detects whether the car is retarding quickly (as in jerking the belt forward by hand) or controlled (as in braking firmly). Depending on the situation it either blocks the seat belt (former) or pulls it tight (latter). The seatbelt is usually pulled tight across your lap to prevent the occupant from sliding under it and wedging their knees/legs under the dashboard/steering wheel. This is why it is so important to keep the lower section of the seatbelt as flat as possible across your legs and not against your lower abdomen.