In posture #3, engaging the liver, Bruce starts to describe some of the ways that the postures are linked to breathing. He continues to link the postures to the organs, hands and feet. For one perspective it is a unity exercise. I’ll note that the way he is doing the postures is part of the jing phase. He even mentions the vascular system in Module 4 video lesson 12.
Like I’ve said in other posts, the breathing is often conflated with chi. If you are following what the postures do physically, you should be getting how the diaphragm is just another big muscle. If you know how to bend and straighten your arm, you should know a lot about what is going on. If you don’t, you’re missing one of the big points. This practice is unifying the body and linking the breath to the physiology of the blood circulation and all of its implications – jing phase.