Earlier, I mentioned that working on structure is Step One in developing IP, and is a precursor to making an aiki body. The first step of that process is learning how to align the joints so that you are not using dedicated muscle to hold your structure together, and so that force can easily be transferred in a clean "path" from point-of-contact to the ground, and back. Then, you learn to feel and manipulate cross-body connections from foot to opposite shoulder, and to manipulate tanden (dantien), meimon (mingmen) and kwa (not sure what the Japanese term is for that - the femoral joint and inguinal fold area) to receive/absorb and feed/project force both from within and that which is provided by a partner. These are not "stoopid jin tricks," but a methodological approach of systematic training in increments to condition and build an IP/aiki body.
Any authentic internal system has some kind of "dynamic stationary" standing exercise to work the foundational structure and connections, whether it's called zhan zhuang, tenchijin or 13 Points, or whatever. YMMV with these, as not all such exercises are created equal depending on the individual teacher, school and lineage; however, the general intention is to work the structure and connections. And as I stated, I have seen people start to develop structure within months, and be able to absorb a push and to neutralize a pull too. On an elementary level, certainly, with low-level stress from the training partner, but even so, it is the "stuff" of rudimentary skills and basic understanding, and will be built upon continually and forever. It should not take years if the person trains a little every day.
IMO and IME, that feeling of ease, balance and freedom of movement comes along after you've learned how to control your structure so that you are acutely aware of its position at any given time, and of any possibility of imbalance... and can adjust accordingly, without tensing up.
I think there is a danger in equating IP with alignment and structure. It is neither in the sense one normally thinks about them, and it is especially not alignment, nor trying to structure the bones of the skeleton. You're not trying to, as we might usually think, "get behind" or "take" force - so please let no one take away the idea that internal power is this, because it is not, and that has already caused much debate on this forum among people who may not appreciate this particular viewpoint.
I made this mistake early on and only started making real progress once I dropped this misconception for good. I had to even give up certain activities like weight lifting (by my own choice) that only served to reinforce this pattern on a subconscious level and prevented me from being able to manifest the ideas despite much effort.
There is a certain form of "structure" that arises from a body that is supported in all directions, but definitely don't think of taking stuff to the ground or making paths or anything like that. There is no one direction or path. You are going from your dantien/hara/tanden/whatever-you-prefer-to-call-it out to everywhere
(that includes everywhere in yourself, not just everywhere outside of yourself - "aiki in me before aiki in thee") and it is this that puts you on the floating bridge of heaven
, not trying to align joints or make a structure with the bones. Again, forget about ground paths or lines or connecting to someone's center or anything that takes you out of neutral and gives your directionality a bias. If someone comes into contact with your surface, there's no need for you to connect with them, because you were already connected with everything
. The ground is not special in this sense - it's just something contacting you, and it is no more privileged than anything else, the end.
Like the surface of inflated balloon, there is a tautness and lack of slack that comes from everywhere on the surface, so everywhere you touch the balloon you feel the integrity of that pressure. But if you were to put a hole in the balloon anywhere, anywhere at all, this nice tautness is gone, the balloon is now a deflated piece of rubber. One little gap in your all-sided support, and your profoundly neutral body is for crap, it is no longer profoundly neutral.
That is in one sense why this is so hard to do, and why it is so hard at first to really understand what the fuss is about - because most likely one has nothing, nothing at all, and doesn't realize it, so one can't feel any absence of the ability. Only when you start to get a little bit of it somewhere, does the daunting task of building that impenetrable surface start to dawn on you... That is, again, the grand irony of it. Someone may think he has something initially, only because, really, he just has nothing and is blissfully unaware.
There are certain conceptions of structure that are certainly powerful and are yet different from this, and they're scattered all over Asian martial arts, but they are not the kind of neutral body that you need as the basis of aiki, so don't make the mistake of conflating them and presenting this idea, like has been presented elsewhere, that they're all the same or somehow equally interchangeable.