Seat belt pretensioner is an addition to the standard locking mechanism. While original locking mechanism prevent seat belt from extending further, those equip with additional pretensioner (most modern car do) will pull back instead and tighten the seat belt pulling the passengers in into their seat in the event of a collision.
As for bear bear question on air bag can pop up even on slight collision kah...no it would not UNLESS its sensors are faulty. Usually the sensors of air bag are place in front, slightly behind the MAIN CAR FRAME in the engine bay. Main car frames can be 2 sticks left/right extending to front within the engine bay. Its very strong and usual minor hit will not damage it...it needs a strong collision to squash the frame to the back. And when it does, it will reach the sensors. When its about to hit the sensor or when it does at split seconds, signal will be send to activate the airbags.
So go crash until radiator, also won't reach the frame, go in further deeper it will reach the frame and the strong frame will stop the impact, but impact too great till it started squashing the frame like pancake, there comes the airbag..
Oh...btw, my Civic fully stock, tyre also stock...except for new PS3 replacement at rear, low mileage 30K. City has some very minor cosmetic mod but no engine mod, just bk, 4point strut, VS, photo conductor cable and Hankook V12 tyres, over 90K mileage liao. Great enuf for daily drives but wife cheong kong, so now driving PE for daily working trips..
Not bad.. Ada PS3.. hehe..