I've not seen the Tomiki unmoveable arm demo, and have not tried it, I look forward to Mike's report back. It seems to go against everything that I (as an aikidoka) would naturally want to do, i.e. when being pushed at right angles at the wrist, I'd want to turn on the spot, not stay stationary.
Well. It wiggled and it wobbled but it pretty much stayed put. The tester was someone who outweighs me by about one third. While it wasn't stationary nor solid as a rock it was, hmmm, well rather difficult to move as opposed to totally immovable.
I suspect this is for a few reasons. One I'm not a solidly built guy so there is slack in my tendons. I found that when the force was applied from one given direction I could handle it, it was the changes in direction that caused the wobbles. I suspect this was force being loaded into my arm and onto the tendons which then released said force when the tester changed direction.
The second reason is. I'm not really that good at this type of stuff.# and need more practice.