Most of us are probably familiar with vehicle airbags. They've been around for more than 40 years and became mandatory for all new vehicles during the 1990s. Some of you, however, may not be familiar with seat belt pretensioners. This device was developed as a way of taking up any slack that may be present as a result of a loosely fitting seat belt.

During a collision, an explosive charge mounted to the seat belt buckle would force the buckle to retract, thus tightening the belt against the occupant. The key word here is "explosive." It's not a good idea, and may actually be illegal to just throw one of these things away with the explosive charge still in place. This means the technician gets to have a little fun when making a repair of this nature.

I don't remember anything about the vehicle I was working on at that time other than it came in for an SRS concern that required a driver's seat belt pretensioner replacement. Since I had to blow it up, I figured a video would be a fun idea. You can see how the cable that holds the buckle retracts instantly after the explosive charge is detonated. Take note of the before and after positions of the buckle.