Ok....here goes from a western physics perspective.
The natural "line of gravity" of the body falls through the middle of the body from the crown of the head down through 2 vertebrate of the neck through the middle of the shoulder joint, then through the lower vertebrate and so on.
In internal arts the sacrum is tucked and the spine is straightened so that the line of gravity falls directly from the top of the head through the spine. This lets the muscles around the spine relax because the bones of the vertebrate and connective tissues are holding up the structure.
Ok...so this is one axis of the "dan tien."
In addition the human body has a "center of gravity" located 1 1/2" below the navel and about half way back (sound familiar.) So lets imagine that you shish kebab'ed the human body through this point either from the front or the side and suspended it in the air. This means that the upper and lower body would be in balance. It would not "flip over."
So those are the other two axises of the "dantien."
The goal of internal arts (aside from fighting, of course). Is to maintain this "center of gravity" at the dan tien. And also to keep the spine stretched out along the "line of gravity." (A side note: This is why a big buff upper body is undesirable, it makes you top heavy and raises your center of gravity. Internal weight lifters take note.)
This is the most relaxed and stable configuration of the human body.
It also means that in WuJi stance the body is doing a balancing act along these axises. WuJi is stagnant and yet not stagnant.
This has several consequences that help explain many of the seemingly cryptic messages in the classics.
For example, if you lift the arms and tense your shoulders your "center of gravity" suddenly comes up. To use the shish kebab example again, this time shish kebabed from the side axis. If you lift the arms and tense the shoulders your center of gravity suddenly comes up. You would essentially flip upside down.
Anyways more to come. But these are some pretty interesting revelations.
Oh BTW, I'm not saying this explains everything in Internal MA's but its a good starting point.