I certainly don't disagree that spirtual practices could facilitate non-resistance. On the otherhand, can't one simply relax and go along with movement (or not go along with movement as their partners interia would only have to overcome their bodyweight and not their musculature pushing back) and "not-resist"?
1) Statements like 'It has to be felt' sound logical yet can be completely misleading.
2) To be understood it has to be done by the person, not felt by the person.
3) On feeling the effect of someone using non-resistance you get many different responses from different people and different conclusions. Thus feeling it is open to great misunderstanding. Conclude nothing until you can do would be my advice.
To be sure there is some subjectivity in what people feel. When referring to what Ueshiba could do, when viewed in light with practitioners with similar skill sets, they are able to replicate a vice like grip, or a ghost like feeling etc as applications of particular skills/principles rather than widely disparate interpretations of a sensation. That is to say that a person demonstrating an unmovable vice like grip is a demonstrating a different thing than when they demo an absence of feeling and feedback.
There is some truth to point 2, in that in order for it has to be felt to be understood to actually work, you need someone who can actually do it, to be able to do it to you.
Anyways, what is interesting about this sort of thing is that when you get it right, you get....... an absence of feedback. That is to say you don't feel any resistance whatsoever, from yourself, your partner or anywhere at all. I'm not completely convinced that this is the result of a spritual practice.