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the belt is designed to go slack, just enough for you to move forward and roll your pelvis back into the seat, preventing the submarine movement under the lap belt. The tension is controlled with sensors and gearing. A harness is designed to do nothing but stay put, which is why it needs to be locked down tight, which then opens the HANS can of worms. I don't suppose you'll be wearing one of those to work and/or the grocery store... ;)
WARNING: I lack industry backing

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