'low over' is very different from 'below'. Here it simply means that the seat belt should be close to the pelvic bone. Compare We flied low over the ocean.
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chivalryThe seat belt should fit low over the pelvic bone.below the pelvic bone can mean underneath the pelvic bone, in other words, inside your body. A surgeon may need to cut below the pelvic bone to correct some problem with your anatomy.