Hi Alan & Near:
I agree that top down inhalation is most common in the traditions and ancient writings. The 4000 year-old Vigyan Bhairava has it that way. The Bhagavad Gita has "surrendering the inflowing-breath into the outflowing-breath" -- don't recall if it specifies direction -- both sides claim the Gita, so maybe it does not. Of course, the nuances and pile-ons begin as soon as people begin playing with these things (ida, pingala, mantras, chakras...), ever-seeking a better way, and complicating things along the way. I have a theory that the traditions became "esoteric" not only to protect the knowledge from the inquisitions and the idle curious, but also because the practices got so darned complicated that only a few could master them anymore! Like trying to squeeze blood out of a turnip, you know.
I am not aware of traditions or writings before the Mahasaya/Babaji line that recommend bottom up inhalation, though there may well be some. Of course, if we believe in the Babaji line, it/he goes waaay back to the Gita and beyond.
Well, who knows? Spinal breathing works, doesn't it? And the more simply it is applied, the better, as far as I'm concerned. We've got other fish to fry practice-wise (like deep meditation) and don't need to be tangling ourselves up in the many nuances (and reductions in efficiency) that can happen in spinal breathing, or any other practice. That is why we stick to the barest essentials of each practice in AYP, so we can do it all across the board and not get ourselves all wound up in one class of practice while making less progress to boot.
Someone said, "It takes a village to raise a child." I think it is fair to say, "It takes an integration of practices to raise the divine child of enlightenment." This is one of the key messages of the Yoga Sutras, and we seem to be verifying it here every day. Bravo for that!
The guru is in you.