Su lai is the action of the sides of the waste holding together.

Kai is the movement of two parts of the body moving outward, away from each other. Even as the (say arms) move away from each other they remain connected as if there were a magnetic force connecting them

Hu is the movement of two body parts moving towards each other. In so doing they stay connected and you can feel the energy moving outward even as the two (arms) move towards each other in closing.

With shun yo the chest is back and moves like a vertical wave as well as horizontal. The vertical is the spine, the chest is the horizontal.