This is a weighty topic. When students first encounter the bones, they laugh. Bones don't make sense. I say:
"Bone Training is weird. You enter a strange and uncertain place. With gravity there is no Yin/Yang. How could there be when gravity does not vary? To separate meat from bones requires great and sustained efforts of mental transformation. Your body's state of bony separation never varies. Our expectations are what varies. At this time, to perceive bony cues is virtually impossible."
I have developed the Mental Models so that these perceptual changes are clear and inevitable. One leads to the next and the next contains the previous. Very difficult, but worth the effort. Initially, practitioners must notice that a heavy mind, such as in those who cultivate gravitas, is exactly the opposite of the lightness of mind required to separate bones and meat. If you find yourself in a grim and serious state regarding your NeiJia practice, stop it. Progressing requires that you walk the finest of lines between great effort and lightness of mind. On the other side of Bone Training you will see that the line is not fine at all.