I have seen lots of videos of internal, I like to look at all these things quite a bit, but I wanted you to specifically show me the videos that you think are important.
I don't see anything on the video of Ark that an athlete cannot do. The only thing I can't do as well as Ark on that video is the suwari waza kokyu ho shove. Then again Ark has been doing this longer then I have. What do you think is on that video that an athlete cannot do?
If you can pull off the shiko (sumo stomp) with someone on your back, that would be rather interesting. That one requires very little leg muscle to pull off despite the fact that beginners use their quads way to much which also applies to mabu/tenchijin as well. While I won't go into the mechanics of how the exercise is done other than saying that it requires "being under" the weight your arms and legs and that you don't bend your waist, the version he demonstrated requires making the other person effectively part of your body.
I will lay out how most people usually attempt kokyu ho, I have seen power lifters, MMA semi-pro's etc usually try it in this manner. After that I will talk about how you should be doing it, which goes back to that list of things that IS people tend to do.
The goal of the exercise is not to push the person backwards, rather to pop them predominantly upwards. Given the path of the arms, there will of course be some lateral movement. Once the person is up, then you can manipulate them over to the side via the same mechanic you use to pop them upwards (via the same list of things I mentioned to you earlier) rather than pulling them around with the arms, or twisting the hips with a rotation. It is more or less kokyu dosa. From what I remember Akuzawa sensei saying in the past, this is more or less how they spent their class time in Sagawa's Daito Ryu dojo.
It has to be understood what it feels like when this exercise is done properly. To both people, it feels like no muscular effort was expended. There is no straining of muscles like when one bench presses a lot of weight or arm wrestles. It is completely effortless and even if one was to stop raising the arms at any point, there is no straining of the arm and back muscles, you can more or less hold that position indefinitely. The partner being lifted should not break their grip at any point, though clearly the harder they grip the easier they will be to lift. If they break their grip, the person lifting has no weight to work with.
This exercise does not rely on usage of the shoulder, biceps and triceps to carry the load, or pop a person back. If you try to lift someone from that position predominantly using those muscles, it becomes quite an effort and your partner can feel you straining. If you try using the biceps, the arms tend to move backwards a bit, making the load greater on the biceps. If you raise/lift with the shoulders, the same thing happens as the shoulders wind up being disconnected from the body and are loaded. Using the triceps is a better way which I used for a long time, but is not correct either. Even if you use all 3 together it still will not work properly in recreating the feeling I described above. You do not want to pull the person forwards either. While this may take their balance, now you have more of their weight going straight down on you which makes them even harder to lift!
It does not rely on the muscles of the back either. One of my training partners has significant back problems and can not use his back muscles without a lot of pain. Nor does he do much conditioning so he has to rely purely on skill in terms of manipulating where he routes incoming force. Now I bring this up as certainly conditioning plays a role in Akuzawa's demo, but want to note that it can be done purely through "skill" in terms of using the 7 things I listed earlier. Chaining together muscle groups doesn't work either.
It does not rely on leaning forwards into your opponent and then pushing. If you think about it for a second, if you lean in, it would certainly add something to a push backwards, however you would be in a mechanically compromised position to lift someone into the air at that point. You would likely find your torso being pushed towards the ground as your partner went upwards. If anything you want to go slightly back to counteract your arm extension. My guess is that we see Akuzawa's slight lean afterwards as a result of his partner's grip pulling him back a bit, but it is clearly after his partner has left the ground.
The version practiced by the aunkai doesn't require opening/closing the body either, namely making the torso go from a concave to convex position. Thats not to say that a breath powered version (hey the aikido version does call it kokyu, while the aunkai version is agete-literally hand raise), would not involve doing so.
It does not rely on structure/optimal alignment either, which runs true of most aunkai exercises where you are put into extreme positions where there is really only a few ways to make the exercise work (some of which are correct and some of which are not I have more recently discovered). This is readily obvious by the starting position.
From the position itself, you can't physically get any lower than your partner....
So how do you do it?
This will not be a full how to, as there is a prerequisite understanding of some of the 7 skills I listed earlier, namely "getting under" so that it feels as though you have a center of gravity lower than your opponents. As described above, you really can not get lower than your opponent physically as you can not bend one's legs even further than in seiza. Nor do you want to lean forward for the reasons discussed previously. If you are under someone, wether physically, or in the manner discussed in IS, it is always a lot easier to raise them up!
You do not want to lock the arms, but instead keep them straight. The arms merely make an arc from your lap up past shoulder height. It should have been clear from what I wrote above that the arms are not bearing the load, somewhere else is. For a long time I tried using the lower back and that is NOT right. It could sort of work, but it isn't really correct. The following is my own personal understanding. Other's are probably doing it more correctly than myself. It's more like you put the weight of your hands in your lower crotch, coupled with "reflecting" the other persons weight/energy into the ground while you push into the ground with the psoas. You effectively need to source power from somewhere lower than your partner.
The above mostly ends the how to, though I would be remiss if I did not at least attempt to explain part of what it means to "get under" or "be under" your partner. Chris, I want to help you understand so that you don't think we are trying to play word games with you.
Alex Lee recently showed me a way in which just about anyone can feel what it is like to be under your own hands VERY quickly. He had me basically squat and hold a bo/jo/bokken with my arms outstretched opponents of me. He then rested his weight on it and told me to stand up. Of course I could not lift it and him up with all my muscles straining. What would up happening was that it was as though I was doing a pullup, trying to pushback against his weight with my arms/shoulders and then trying to push with my quads off the ground.
What he suggested I do instead was pretend I was doing the limbo dance and go underneath the bo/jo/bokken (starts to look in some aspects like a power lifter). Now this doesn't mean lean back, as if you do that at this point you will fall over, rather my crotch pushed forwards and I could feel the inner part of the legs and the lower crotch area between the testicles and the anus take the load. I could then effortlessly raise the weapon and stand up. This took me maybe 2-3 times to figure out and about the same for another training partner who only has about 1 month of IS experience.
Now I can personally recreate that under the hands sensation fairly easily now without having something in my hand, it does tend to fall apart though as soon as I touch someone else as I have to figure out how to "get under" that additional weight and make it part of me. When I do have it, it becomes rather easy to move them.