I don't know if I would go so far as to say that I'm actually expressing in-yo in a single arm, but something that I got from Ark and Rob was the idea of kaeshi (returning) within the body. Mentioning the arm reminds me of this because one of the ways Ark demonstrated how he was doing was different than Tohei's "extend ki" paradigm was to explain that for every amount of "out" he was expressing, he was expressing an equal amount of "in".
To make that clearer do (or attempt) the following:
*Tohei/Ki society version: you know the drill, do the unbendable arm trick, arm like firehose! Shoot ki out your fingers! Reach for the $100 bill! and bang, your partner can't bend your arm at the elbow. ---but neither can you without making your arm "empty". So, yes you're very "strong" feeling but you've created a direct line into your center and you've given up freedom of movement and sensitivity. Not good.
*Aunkai/Ark/garden hose version: OK, so now the mental image is that you have a a garden hose that runs out to your fingertips but then is looped back on itself, so all of the intent/water/ki/midicloreans you are shooting out to your fingertips are pulled back just as hard as you extend them out. Focus on pulling intent back along the return pipe since you're probably already used to pushing them out. Now have someone do the unbendable arm thing on you. When you're doing this right, most find that you are just as stable resisting/being unmoved by a partner's attempt to collapse the elbow *but* you are now free to open and close their own elbow at will. The arm is now alive because it's basically a closed system of intent.
And just to be clear, while this post is a reply to Dan's, this is actually aimed at other readers since Dan either knows this or has a different paradigm that he's operating under.
If I remember correctly in my old days of training in the Ki society (mid to late 70s) we used the fire hose metaphor for the ki extension in the unbendable arm demo, but you also needed to extend ki from the one point; and the 'key' here is that as ki is being projected out in any form from the body, you also had new ki coming into your one point to replenish - this gave you a feedback loop of energy that helped to establish stability. I think the real lesson here is the fact that there needs to be an in/yo relationship to establish the internal stability - Ark's example above has in/yo, my example of Tohei has in/yo, and Dan's model of dual opposing spirals has it as well. Now the question is, which model provides for more stability in a static as well as dynamic state.