Pretensioners are safety devices used in vehicles to enhance the effectiveness of seat belts during a collision. They work by retracting the seat belt slightly at the moment of impact, pulling the occupant closer to the seat. This helps to reduce the risk of injury by ensuring that the seat belt fits snugly against the body. These devices are typically part of the vehicle's airbag system and are activated by sensors that detect a crash. By minimizing the slack in the seat belt, pretensioners help to keep passengers securely in place, improving overall safety during an accident.