Its said that when you breathe out you contact the Root of Heaven and experience a sense of openness, and when you breathe in you contact the Root of Earth and experience a sense of solidity. Breathing out is associated with the fluidity of a dragon, breathing in is associated with strength of a tiger. As you go on breathing in this frame of mind, with these associations, alternating between movement and stillness, it is important that the focus of your mind does not shift.
Let the true breath come and go, a subtle continuum on the brink of exisence. Tune the breathing until you get breath without breathing; become one with it, and the spirit can be solidified and the elixir can be made.
A Complete Guide to Chi-gung
by Daniel Reid 2000 (Thomas Cleary Vitality, Energy, Spirit)