I enjoyed your post overall, but have clipped and emphasised one small part, to disagree with, however, I may well be missing your point.
It is my understanding that extending ki and weight underside has a great deal to do with body mechanics. Although they are both mental in their origin, the correct body mechanics must be in place as the vehicle to allow the mind stuff to happen to proper effect. In effect the body must be relaxed, in balance, all movement coming from the centre.
I completely agree with you that the execution is very soft in it's execution, but for this to happen the mind has to have the correct body to back it up. This is particularly apparent when trying to teach the type of aikido that we both do. How many times have you had students who are trying to execute a technique, with softness, with extension, without struggle, but without the desired effect. They may even feed back that everything in their mind is towards that goal, but their body is scuppering their attempt. They engage shoulders when not neccessary, they hold tension in places they are not aware of, they hold their weight in the wrong place, their hands are not connected to centre, their co-ordination is not as complete as it needs to be.
I know you know all this, so it may well be me getting the wrong end of the stick, I don't want to be teaching granny any egg sucking here!
Yes, you are quite right in what you say and that particular point was made without enough explanation I fully agree.
It was however not completely without a reason for doing so. It was partially given for food for thought. I'll explain.
When showing someone how to do it I get them to try with force as hard as they can and to feel what it's like also. In other words get someone or me to do the same when they hold the bokken.
Then I show the difference as you will be aware of.
Next, on getting the person to experiment and practice it will be quite as you say, aligning properly, pointing out through observation the physical reasons for it not working, the mental reasons and then the spiritual reasons. All step by step so of course body mechanics are part of it.
In fact you could say in ANY motion body mechanics are in play obviously. The emphasis however changes completely or rather can change completely depending how confident or capable a person becomes.
For example demonstrating weight underside can be done from so called 'impossible' positions. 'Impossible' when looked at from the perspective of body mechanics or standardly correct physical posture.
I would say Tohei could stand sideways on and one handedly do the same thing for example. In fact no doubt he could do it with a soft hand let alone a bokken with his body facing another direction as he's let's say looking for or turning to meet another attacker let alone for demo purposes.
These things are all not a matter of conjecture but rather of individual capability. Degrees of, I would say.
Only a few months ago I was doing a similar demo in class, albeit against a jo, showing the difference to a student. Showing a hard strong cut and then he had to explain to the class what it felt like apart from them seeing the effect themselves. Then I did it with weight underside and with the intention of knocking the bokken straight down. It worked and he picthed foreward etc.
As it was all finished someone asked me the question what if. Oh no, another what if. This time it was what if you did it like you have said before with absolutely no intention to knock the bokken down but more spiritually.(They are used to the way I talk) Well, it was one of those times when you're asked not because they want to do it but to see the difference.
I found myself explaining first the differences in me mentally, physically and spiritually as to the approach needed. I jokingly said 'I suppose we would have to call this super soft.
Well, it was indeed super soft, meditative, still, but the effect was rather unexpected. The jo broke clean in half and the student was face down on the mat.
One of those times for quiet reflection. The purpose of the demo had been successful, namely potential. That added something or maybe lack of something, no doubt both, was for me to understand in myself.
The overall point here is that things may seem 'out there' but as with many so called spiritual martial artists who to me are too airy fairy the discipline is what's missing even on the spiritual side. It still does take a correct alignment of all three, spirit, mind and body. An awake spirit, a still mind, a relaxed body to put it simplistically.
You know what, I believe that many equate the spiritual side as passive and therein may be the misunderstanding.
Hope it doesn't come across as rambling.