In an accident the seat belt must catch the occupants as early as possible and therefore needs to be worn tight. Due to the occupants' clothes and the looseness of the automatic belt retractor this is not always the case. This effect is called slack. Pyrotechnically-triggered belt tensioners compensate for the slack by tightening the belt on impact.
All SEAT models come with belt tensioners as standard which, in combination with airbags, provide the best protection at present against accident injuries.