Center. Hara. Aiki-heaviness. These days, I’m thinking they are all the same thing. How do you develop it? I don’t think it’s by doing aikido technique as ‘tori’, regardless of imagining that you’re doing it from your center. I think, as related to ‘normal’ aikido practice anyway, it has a lot to do with dealing with a lot of full-body, from-the-center power, and this means it is done as ‘uke’, specifically receiving it from partners who are skilled at sending power to your center.
Part of the reason one does not develop this in the ‘tori’ role is because it is rare to receive so much power from ‘uke’ that you might be overwhelmed and need to receive/connect that power with your center. ‘Uke’ would have to be quite skilled and be attacking and responding during the technique in a way that is on the brink of overwhelming ‘tori’ while still allowing ‘tori’ to perform the technique, which means to be overcome. This is inherently difficult.
Closely related to the above, ‘tori’ only is subject to not being able to execute the technique. There is usually no risk of the ‘tori’ being overwhelmed or toppled. That is, there is not so much for ‘tori’ to need to receive. For ‘uke to access ‘tori”s center while in the various, contorted positions is difficult. Basically, it would be “doing aiki” while physically appearing to go through the motions of ‘uke’.
I think that the “aikido way” of developing center is by receiving a lot of center power from a skilled ‘tori’. First, the type of power is not simply “arm power” or athletic power. Second, it is the skilled ‘tori’ who can access ‘uke”s center. Lastly, as ‘uke’, the objective is about receiving power, not necessarily overcoming the other person. Receiving and reconciling power.
There is one more component, which I believe comes after the stage when a person can properly and safely take a lot of high-level ‘uke’. Without this component, a person may appear to be a very good ‘uke’, but at this stage 1) it’s still only about receiving and giving power at a bare minimum and 2) ‘uke’ does not generate, or give out’ much power i.e., they only give out enough power for ‘tori’ to manipulate; ‘tori’ is never at any kind of risk. The component in question is to continually attack and fill the space of ‘tori’ while having less and less tension. The difficulty is that this must be done while still being manipulated by ‘tori’ and allowing the ebbs and flows, and changes in direction in a regular technique to flow. When a person can attack and threaten ‘tori’ but ‘tori’ can still manage, then that ‘uke’ is developing center that is both giving and receiving, is soft yet centrally solid, and mobile and responsive and not wedded to a narrow range of manifestations/actions/patterns.