I would add to that, that the manifestion of yin and yang can be shown in a single arm movement. I do this at seminars where I have someone put force into my arm and I move and they move. I then do the exact same movement now deviod of yin and yang and their power comes in or I have to use muscle. Then....apply yin and yang...I move, they move.
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.