Nah, I haven’t read any books on MBTI theory. But I think I get it now. From what I’ve read of Jung he was pretty big on ‘cod-dialectics’, i.e. dichotomies, balance, ying yang, male female, and all that spiritualist stuff that was popular in Weimar Germany and later Hippie America.
So I get it form his logic now. The auxiliary is the ‘counter/reaction’ to the dominance of the dominant.
So it’s sorta like if all the ‘energy’ (or whatever it is) goes into one function, it leaves a vacuum elsewhere that causes a reaction to balance it out (homoeostasis, or some sort of balance / reactive logic).
So to balance the dominant function energy has to go to another function, which is distinct form the dominant one, hence why it has to be from the opposite P/J and I/E square.
So Ni-doms can’t be balanced out by Se because it’s still a Perceiving function, and it can’t be balanced out by Ti because it’s still an introverted function. So a Ni-dom can only be balanced out by Extraverted Thinking or Feeling.