A seat belt pretensioner kind of resembles a gun. Under the seat by the center console there is a roughly 6 inch tube. Inside the tube is a slug with a pyrotechnic charge sitting behind it. The slug has a cable attached to it in such a way that when the slug is propelled down the tube the cable is retracted inside. The cable then is anchored at the bottom of the buckle. When the pretentioner fires the seat belt buckle drops down several inches.
Another kind of pretensioner lives in the B pillar where the seat belt spool resides. There are two general mechanism here but the idea is the same. The side of the spool has a cog wheel. Next to the cog wheel is a tube the directs something at the cog. That something can be a set of balls, that engage the cog like a gear and are propelled down the tube with a pyrotechnic charge. The other something is a rod with teeth cut in it. The rod is propelled down the tube it engages the cog. In both cases when the cog is engaged the cog will rotate the the set belt spool backwards several revolutions. This pulls the seat belt back into the B pillar.
All the mechanisms work along side the air bags. When the air bag computer decides it will deploy the front air bags it will then check if the seat belt is buckled. If it is buckled then before the air bad is deployed the pretensioner is activated. This is done to make sure that the occupant is pulled away from the air bag while it is deployed. While the air bag is deploying it is hard as a rock, only when the air bag finishes deploying does it become something soft to fall in to. The system is timed in such a way that when the pretensioner is done it stops holding the occupant back, by this time the airbag has finished deploying, allowing the occupant fall into a soft air bag.
The system has been on cars for about 15 years. Today almost every car is equipped with this system.