Can you talk more about invisible shoulders, Graham?
Sure. From the practice of weight underside and relaxing completely you find your shoulders disappear, become inconsequential. The same also applies to sword work.
So I find that even the thought of using shoulder has a subtle detrimental effect and I would say the greatest detrimental effect is on centre line.
So in conclusion I say they should have no resistance in them and so when you move someone back who is pushing your shoulder the the illusion is that you are using your shoulder. Actually I let go of my shoulder, turn back around centre line.
A good exercise I get people to do is having someone push through their shoulder. They are to let it go, not resist, and allow the upper body to rotate around centre line. This can then be moved on to letting it go and allowing the upper body to rotate and then rotating it back in the opposite direction. (back to original position.)
The only thing that get's in the way is the persons own mind and thus more masakatsu agatsu.
The main principles are thus letting go and keeping centre line (neutral) and acceptance (centre).
That's my boiled down explanation without going into energy.