Thanks for the reply Walter
One thing that interests me about IP practitioners is their attempts to put it in scientific terms, since in aikido it seems to be encoded in animism. But regardless of how it is explained, I would recommend that if you are able to find someone who can do it well, you should ask them if you can feel their arm while they lift a bokken.
It would feel different, the array of musculature that's used is generally different, and it's employed in different ways (along with some additional activators and controls) - but nobody who's really studying IP has claimed that no muscles are used at all.
Still, the difference is big enough that it has confused physical therapists who were expecting something else to happen - and that's partly the point, that confusion is itself a big advantage.
The scientific stuff is interesting - to a point, but too much of it doesn't really seem to help in actually doing this stuff. The old standbys of visualization and imagery that have been used for thousands of years are usually the most effective in that regard. Phil Jackson and the Eastern European Olympic coachs seem to have reached similar conclusions, in many ways (in terms of the value of visualization, not as related to IP).