Unfortunately, I really believe Kelly wants you to feel stupid. Every analysis he presents is overly complicated, described with obscure non-standard jargon, and explained too quickly to be grasped even if it were presented in a normal tempo in grammatically-correct English, understandable, logical, simple, and correct, which it never is. OF COURSE the lumbar extensors establish and maintain lumbar extension, because that's what they're for. OF COURSE the abs and obliques reinforce this position isometrically, like a cylinder, because the muscles on the sides can't actually move anything when they work together, and the abs in isometric contraction just pressure up the gut, holding the spine rigid from the anterior. And OF COURSE the hip flexors have nothing to do with it, because 1.) they are tiny muscles -- how much weight can you put on your knees and raise to your chest? 2.) hip flexion is easily provided for by 405 on the bar and you pushing on it at 402, and 3.) LUMBAR EXTENSION is the primary problem, not HIP FLEXION anyway -- as the weight comes off the floor, the problem is keeping the back flat (at least for the people Kelly works with), and staying in hip flexion means the hips did not extend, hip extension being a critical feature of any successful deadlift.