Hi Mike,
A couple questions:

So there is no groundpath usage whatsoever in yin qi/ki development?
Could it be the directional aspects relate to the relative directional traits of "heaven and earth?"
If there is always a bit of yin to yang and vise versa (as I thought I understood the model to show, at any rate), how can we have any "path" that is strictly yin or yang?
My weak guess is that Mark is refering to the sensation of gravity (downward) as the one-directional groundpath (Is that right Mark?).
Thinking of my earlier thoughts about vectors relating to the vertical axis, I can see how any point along the "pathway" wouldn't necessarily pertain to any particular direction, since you have equal and opposite vectors always present, which would seem to make sense (or at least appear to do so to my mind) regarding your remarks about the non-directional quality of groundpath.
I keep struggling with the right wording to describe what I'm trying to convey so I'll leave it as it is and see what I get.
I'd appreciate anything you'd have to throw my way on this!
Take care,
Matt
ps- my current mental efforts are to reach up and down (yin and yang qi effort respectively?) at the same time...at some point I feel a kind of balance in which I feel very "floaty," though not in an upward direction...much like a hot air ballon which has attained pressure equalibrium with the air around it, being pushed neither up nor down but resting in between.
Though of course, I'm not even close to knowing anything so...probably just hot air. Still, interesting to contemplate.