I've had a little extra time on my hands lately, and I could very well be way off base, but the more I read here on this topic, the more I have a sneaking suspicion that a part of aiki, (maybe the foundation) is the opposite of muscle contraction.
I wonder if there is a way to train to move with complete, whole-body, muscle extension. Perhaps that's what is meant by being "connected"," fascia", "long muscle"," softness in hardness/hardness in softness", etc.
If a muscle, say the bicep for example, is contracting against some resisting object and the object suddenly gives way, what happens? Sounds like kuzushi to me. What if, instead of the muscle fibers of the bicep contracting to bend the arm against resistance, the muscle fibers of the triceps were somehow extending to drive that same movement? If the hypothetical object suddenly gives way while the triceps was extending, I wonder if the reaction would be the same. Wouldn't the movement be counter-balanced by the extension going in the opposite direction? Now, if I could train to move my whole body with muscle extension, wouldn't I have superb balance? How awesome would it be to be impervious to kuzushi?
Sorry for rambling. I just had to get that thought out of my head, it's been gnawing at me for months now.
Personally, I wouldn't explain it that way ... but in theory it's close to how I'm looking at things. Mostly, I wouldn't explain it that way because I'm trying not to activate bicep or tricep muscles at all, even together. Course, I might be trying to get them to the microsecond pre-stage of movement.
In other words getting my brain to send the signal that I want to pick something up. My hand grabs the object and at the very microsecond that my biceps would physically contract to lift, I don't let them do that but keep them at that pivotal point. At the very same time, I'm getting my brain to send the signal that I want to push that object down and at the very microsecond that my triceps would physically contract, I don't let them do that but keep them at that pivotal point. Contradictory forces using both biceps and triceps, without the actual physical contraction of those localized muscle groups. But, yet not really the actual physical extension of them either.
Same concept with the ki unbendable arm trick. You imagine a flow of water going out your arm. What really makes that work? You're sending a signal to the brain to "push" something outward but not really letting your muscles contract physically to finish the job. Except, as I mention, you have to have contradictory forces going, so outward is only 1/2 of the "intent" needed.
How do the arms tie in to the body? The shoulder area. Which, for me, is a huge obstacle because of muscles wanting to physically fire and get in the way. Once localized muscle groups (for example pecs, or biceps or quads) fire and physically contract/expand/whatever, that restricts "ki flow" as some would say. Intent outwards and inwards, from and to the spine. The shoulder joint, in that contradictory force does not get pulled outward nor does it get jammed inwards by force. That shoulder joint becomes the connection point for the arms to the upper cross.
Rinse and repeat with entire body. Structure.