Its been gone over many times in the threads. We -meaning dozens of guys who are now training-sometimes tire of repeating the details. In fairness your question is a solid one though.
The general idea is that you need to work on your body. There are ways to train that strengthen your held balance- your central equilibrium or zero balance state. Think of it like being suspended between managed lines of opposing forces you control. Because they support each other, and leave you stable and relaxed anyone who interacts with you becomes part of a line of force you already manage. Once your body is trained you can begin to use it to make aiki happen on contact.
Some wierd effects that are consistent with an idea folks may have of aiki-that being blending: I can send a line of force that wants to push me up and over 45 deg to my rear by actually agreeing with it (not moving on the outside) with a managed line of force in me that is natural while sinking down through me on its opposite. There is a whole other aspect of "polishing the mirror" but thats not for here. Thus they wind up pushing down, while they swear they are attempting to push up and over. This works great for arm to arm grappling where they have to close the distance or shoot.
You train to failure statically and then of course in waza then in motion or freestyle. Always the same; static, waza, motion.
Some of the benefits are when you touch someone they are like a spider touching your held web. You can find their center instantly and control it. If they move, you move. There are so many side benefits; speed in motion due to lack of slack in the body, very substantial striking power, and an extremely difficult body to throw or lock, and heightened sensitivity to positioning and change.
After all of that, there are ways to use your newly trained body in different martial applications that are fairly common in the arts. The art doesn't matter. It is your choice. Kata is NOT the way to train aiki, Kata are the result of aiki in changing and controlling force.It takes years to do.
Thats just some very basic concepts to consider.
It expresses itself in any grappling art- where there is an on going and rapid interchange of force and postional change coupled with power strikes, rather well.
The reason you will find so many converts is that it is the heart of what aiki is. Once you feel it-there is no debate. The ones who debate here have not felt it and some are just bitter that it is stated so flatly and confidently. Where I might disagree with you about aikido being complete in that
a)all of this knowledge is there
b) Where there is some ,whether it is taught
What we are doing is to teach openly. People train their bodies specifically for aiki in a manner that is clear, decisive and demonstrative. You may want to take note that many are reporting results from still short periods of time. More good news? We actually have students with demonstrable skills that you can find.
There are very few left over, naysayers here. Most rational people have looked past us and fairly and intelligently accepted the words of so many of their own at face value. A few are hanging on and denying the consistent testimony of many experienced aikidoka and debate for debate sake.
Good luck in your training
I understand what you are saying. I respect your disagreement with me. I was taught what you laid out to be something you develop over time and practice. That is the hard part. That is the explanation for why everyone is not at the same level.
We are talking about a natural law, that I don't know the scientific name, but it is universal and functions Aikido and other stuff, right. If you think it isn't in Aikido then it can be applied.
Training the body then is a matter likened to conditioning in sports? Conditioning exercises practice for the specific sport is what I am saying.
I do agree that if it is about conditioning that enhances performance , with the addition of the natural law then there will be noticeable results, more so if without any of that. Applying scientific principles and laws will only enhance Aikido. I see that those laws already exist in Aikido. I see Aikido as a adventure to explore, a universe. Sure it isn't easy to do train this way, and sure some people like me don't have the time or resourse to be that devote, but really it is the journey that is most important, and not the journey's end. Peace.