Seatbelt tensioners are life savers. They're a little explosive charge under the chair near the seatbelt attachment point that yanks hard on the belt to squeeze you against the chair while the car comes to a stop. They're super powerful and you don't want to mess with them unless you know exactly what you're doing (see also: airbags).
Interesting! My recollection was that it was a non-explosive tensioner, but maybe I'm wrong. Thinking about explosive tensioners reminds me of an EOD tool I recently learned about – the rocket wrench. https://www.youtube.com/watch?v=11qa5LFB6zI [1:36]
