The pelvic support prevents hyperflexion of the spinal cord which can irritate or damage surgical, or painful sites. (ivghospitals.com)
The violent forces associated with sudden impact can cause hyperextension and hyperflexion of the soft tissues, i.e., sprains, strains, whiplash and similar injuries. (yourlegalguide.com)
Using a little more trunk extension is okay as long as you are not compromising your postural position (i.e. don't go into lumbar hyperextension or hyperflexion). (kingofthegym.com)