Overweighting students to a frightful degree is all too common, but hopefully it is starting to be phased out. IMO, the reason for it has nothing to do with safety but it is related more to an age-old instructional strategy that is gradually going the way of the dinosaur.
Scuba instruction began before any buoyancy control devices existed, including the wet suit. Research on this history shows that as a result, skill instruction began with students kneeling on the bottom of a swimming pool. That approach continued to be used by almost all scuba instructors until recently. When wet suits and then BCDs became part of instruction, weights had to be added to keep the students firmly planted on the bottom so they could do the skills without bobbing about.
I led a team of instructors a number of years ago in writing an article on a different approach to instruction--having students from the very beginning of class neutrally buoyant and in horizontal trim. We convinced PADI to publish our article in their professional magazine, the Undersea Journal, and PADI asked me to provide pictures that compared the two instructional strategies. I first posed for all the neutral/horizontal pictures, dressed as I normally was when instructing, which means I was overweighted by several pounds so I could better manage students. Then I posed for the pictures while kneeling, something I had not done in years. I had to double the amount of weight I was wearing to remain stable while doing the skills! That is because of the high center of gravity while kneeling--you tend to tip over if you are at all buoyant.
While I was negotiating with PADI over the wording of the final draft of that article, it was obvious from our give and take that they had never seen instruction done that way and were not convinced. Two years later, they published new standards and recommended that all instruction be done neutral and horizontal, in keeping with our article. They eliminated all pictures of people doing skills on their knees. I recently had another article on this topic accepted for future publication, and I had to go through negotiations with the same person as before. This time there was no give and take on the content itself--we only discussed wording. That content included ideas that were not included in the first article because they were then considered too controversial and too adversarial in relation to the old concept of teaching on the knees. PADI had obviously seen enough since that first article to be convinced it is the best way to teach.
Unfortunately, although I can assure you that PADI headquarters is now convinced that teaching on the knees is not the best way to go, they did not forbid it. Instructors are free to continue that way if they wish. I am sure that the overwhelming majority will continue as they always have. Amazingly enough, I know someone who recently got his instructor certification through a huge program that certifies hundreds of instructors a year. They told him there were two ways to do the skills, and that the neutral way works best, but they still required all instructor candidates to perform all the skills heavily overweighted and firmly planted on the knees! Why? They had been doing it that way for many years, and they knew that if their instructor candidates performed the choreography of the skills as they taught them, they were certain to pass the instructor exam.
If you are horizontal you are forced to use your buyancy device and/or lungs to adjust your trim and moving your legs will only move you in the horizontal plane.
