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.
This is a demonstration of proper energetics not waza. It is the energy that one has when one presents ones hand in training for the partner to grab. There should be structure there, not just a hand hanging out in the air. That is a demonstration of that structure.