I hope it didn't sound like I was trying to quote Mike or explain what he thinks. I was trying to frame some of what I was thinking relative "something" he said somewhere (and I have no idea where to look for the quote).
Mochizuki Sensei specified that ki itself has both yin and yang and that aiki to can be either yin or yang. In either case, I think it would still be a combination of yin and yang.
Please comment more on this.
I can only offer my own understanding, from my own practice. In and Yo (Yin and Yang) are qualities or states, not energy (Ki, Chi) itself. You know that In and Yo are complementary opposites that cannot exist without each other; they are always present in some form of balance. They can be maintained in a steady state, or they can fluxuate with the conditions, but where it comes to manipulating energy in the human body, there is no "all In" or "all Yo." There is either a neutral state of equal expression, or the expression of more of one quality, to some degree, than of the other.
So, the aiki we are creating can express more of the qualities of one than of the other. That would be my interpretation of "In/Yin aiki" and "Yo/Yang aiki. Thus,""In Aiki" would be aiki that has more of the "In" qualities -- aiki that draws and sucks in, and "Yo Aiki" would be aiki that projects and expands out. The corresponding parts of the body and processes that manipulate ki/chi/energy this way would also be those associated with "In" and "Yo" and used in the same balance as the aiki they produce and express.