From the standpoint of the spine - what I care about is that I'm settling such that the upper body is fully supported by the lower body so that it can transmit the power generated by the legs and transferred by the middle. The spine then has a management role from the point at navel-ish height and the point where the band under/across the shoulders meets (though the latter tends more towards conveyor here, too). Initial efforts have the breath "packing" the body with a stretch that turns to pressure (which eventually you will manage via intent after a LOT of conditioning over a LOT of time) that will let you transfer the lower body power throughout your body as one unit moving all of you at once (see other posts on the six-directions, ground/gravity stuffs - all relevant regarding the "what" power and combinations in which its used).
There's more, but that's the entree' point at which the initial conditioning work should pay attention towards and build to. It will be hard to get too far unless you someone knowledgable showing you the steps and keeping you on track, but that's the logic.
Thanks Budd, and thanks Marc for the original excercise.
Here are some thoughts I have, take them as you will.
What about, when one exhales, the area from the diaphragm downwards doesn't "rise", rather the pressure sensation created in the lower body by the inhalation/breath is conciously maintained? This pressure allows the spine to "rise/stretch" naturally.
The reason I asked about the spine was because the upper body, as I see it, should also include connection of skull and spine. If the skull is correctly balanced, the breath should do a lot of the stretching naturally.
Does this correspond with what you mean by "packing"?