I think your confusing imagery with what is actually happening. If you are not actually below them, in some way, you are not horizontally more stable then they are. Nature rule Hunter san, not mine
. When you are feeling with your intent, what you are actually doing is finding a "sticking" point in their body, then you drive under that sticking point, from a lower position. Because their whole body is connected, once you find a spot to raise them up, their feet will go as well.
Actually, finding a sticking point isn't required, nor do you need to get physically lower to ever get under anyone.
Here is a relatively simple exercise, that most people reading this should be able to figure out within a few reps. This is of course not the whole story, but relates to the shoulder stuff I talked about earlier and is only a portion of how I would execute kokyu dosa/kokyu ho. This is still a "muscly" way to do the exercise, but will develop some other things. For this super simplified version, don't use the lower torso musculature, hips or legs at all to push back, other than to keep your weight going straight down. I don't want any "additives" of any kind to complicate things.
You are going to start off in a standing position as it is easier to do than from seiza. Stand with your feet parallel and arms at your side. You can either have a partner grab both wrists or one much like they would in seiza. Extend your arm downwards without physically dropping or bending over. If you keep on extending downwards, like you are projecting your elbow downwards, you will find that your arm will eventually not be able to go any further down, but starts to rise with the shoulders remaining down. Your partner will begin to pop upwards and backwards and neither they nor you will feel any resistance. You don't want to fall forwards as your partner's weight goes out and back.
No explosion of power is required, in fact you can do this very slowly whether standing or in seiza and you won't feel any resistance.
You do the exact same thing in kokyu dosa, but its a bit more challenging because the arm is bent. This is why you project it out from the elbow and the position of the forearm/wrist becomes largely irrelevant assuming you know how to connect the shoulder to the body (which I haven't discussed at all).
Whats going on here? Well, by taking their weight/push and having it go downwards (and adding to it a bit), the resultant force goes upwards. From your partners perspective, you are pushing from underneath them
, even though physically your body has not dropped at all. You don't find a sticking point at all, you are merely adding their weight/energy to your own and thus creating aiki.
Someone might say, "but Hunter, didn't you say, for internals you don't want to do the whole, you push, I pull thing?" Well, first of all, the way I discussed above was more of a muscle based version, I'm fairly confident that others reading this know how to do the same thing. This isn't dantien driven movement, however you are using your partners weight to cause them to move themselves.
More importantly, I said you want to keep your weight at all times pointed straight down. Forcing my partners weight down by integrating it within me does exactly that. Further you maintain that, by keeping yourself from being "pulled" forwards (as in off balanced in a forwards direction) as your partner moves away from you.