It puts the baby's pelvis in a posterior tilt, which facilitates lumbar flexion, not extension. (articles.chicagotribune.com)
You may be completing kettlebell swings with excessive lumbar flexion and not feel it in your back as you do so. (onnit.com)
This one exercise addresses all the major core stability concerns of runners: avoiding torque / twisting, avoiding lumbar flexion, avoiding thoracic flexion, and avoiding pelvic rotation. (onnit.com)