that is a primitive imitation of John Parillo's invention! He has an actual machine/platform made just for that along with a special harness you wear. That machine works awesome. The guy in the video wasnt going low enough. I remember being able to break parallel on John's easily and never had any excessive strain or injury. He used to come train one of his powerlifters at the gym I trained at once per week. When they came I would do legs with them and this is one of the exercises we would do. the guy's dad cut us special 200 lb plates to use on it so we could strap on more weight. Steve Hampton, the powerlifter, would strap on as much as 800 lbs and do reps. I worked out with as much as 600 lbs. Cant beat this exercise in my opinion. No stain at all on the lower back. At times it felt like it was going to strip the muscle right off my bone, but never one injury. Very intense.
John loved to have us do forced reps, and we would have 3 guys pulling you up once you hit failure. Would do many of these force reps after you had hit failure ot the point where you were seeing stars.