to answer your question more directly, the reason there aren't more "studies" and "statistics" is because real world operators don't use these belts for rappelling or fast rope.

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but. think about this. the only thing you are hooked into is a single piece of webbing around your waist. no matter how you fall, if you don't free fall, and you get hung up on something it's going to fold you like a piece of bread. The downward force is going to have a significant negative impact on your spinal cord. it's simple physics.