Seatbelt pretensioners tighten up any slack in the belt webbing at 0.020 seconds after an impact is detected by the ECU (Electronic Control Unit). This restrains an occupant as early as possible in a crash and reduces the load on the occupant in a crash while restraining the passengers in their seats more effectively. This type of prentensioner is typically located on the inboard side of the first seats. However, several vehicles I have seen during the development process have this type of pretensioner install on the B-Pillar and covered by a plastic trim panel. Look for more information about this once the vehicles hit the roads.
The first video below shows the operation of the prentensioner by retracting the buckle and the second video shows the movement of the belt.