Shawn, and Michael's last post, seem very clear and right to me. I think the tendency is not to do it naturally in the beginning. Just like dancing. Getting out of the forced or awkward stage. I certainly don't want to confuse this , but one can define a difference between filling the abdomen/lower lungs, and upper lungs. A two stage filling of inhale, that really is full. Sometimes I focus on filling the lower part(in a very smooth relaxed manner), then continue, filling up the upper lungs. Like filling a bucket from the bottom up. I learned a form of medical qigong once, that when working on the lungs, separated or defined these two areas of filling up, lower and upper. One of the exercises was done in stages. The first stage just filled up the bottom. When you got to doing both, one was very aware of how full you could fill up on the inhale. The exercise focused on "filling" the lungs to expand and energize them, beyond the normal capacity usually experienced. I find, defining this lower and upper helps create a fuller breath.
I also find that being aware of pullin up the huiyin area, coordinates nicely , with pulling in that area below the navel. I have seen qigong exercises that just focus on reverse breathing. The form they took was very similar to the breathing of the universe exercise in level I. I think that exercise is a great one for practicing reverse breathing, as your hands move apart and together.
