The vast majority of this material must be taught in a small class because each person needs a fair amount of feedback and interaction with the teacher. That's why it's not very well known or understood. It's this lack of personal attention which leads people to keep doing brutish things like sit-ups.
I thought I would offer one image which my student have often found helpful.
When the arms lift in Heaven Earth style qigong (or Baguazhang circle walking or numerous other internal arts/movements I could point to) the protuberances (Spinous Process) on the spine starting in the lumbar region move upward. This is true whether you start with the arms moving to the front or to the sides.
This movement matches that of a flying fish jumping out of the water. When the fish jumps its dorsal fin (the one on its lower back) goes up and forward (towards the head). The dorsal fin on a fish is actually the spine, it is simply a spine with long spinal protuberances (Spinous Process).