Pretensioners preemptively tighten the belt to prevent the occupant from jerking forward in a crash.Like airbags, pretensioners are triggered by sensors in the car's body, and most pretensioners use explosively expanding gas to drive a piston that retracts the belt. Pretensioners also lower the risk of "submarining", which is when a passenger slides forward under a loosely worn seat belt.
source : WIKIPEDIA