This is Master Zhang's verse on Can Tong Qi:
The clever usage of Neidan is in-line with the natural order of Qian (乾) and Kun （坤）,
Qian and Kun breeds the Five elements （五行),
The natural development of the Five elements breeds the cycles of Life and Death,
The turning back of this natural course breeds the elixir to Immortality (丹).
One comes from Nothingness,
One then splits into Two of Yin and Yang (两仪),
Two combines to offer Four (四象),
Each of the Eight trigrams can evolve into one another, like grandfather like grandson.
The logic of such change breeds infinite possibility,
Good fortune or bad thus come from such successive changes.
The common people don't know this change happening daily,
Only the sage understands the logic behind.
Therefore I-ching reviews the theory of change of Qian and Kun,
therefore the author (of Can Tong Qi) used it to signify his (Neidan) teaching.
Pi（否） and Tai (泰） are used to signify the rise or fall of Yin and Yang,
Tun (屯） and Meng (蒙) are used to signify changes in the early morning and late evening,
Kan (坎) and Li (离) are used to signify male and female, water and fire.
Zhen (震） and Dui (兑） are used to signify dragon and tiger, Po (魄) and Hun (魂).
Keep to one's centre (收中） is expressed by the good omen of nondistinct-yellow-clothes (黄裳元吉),
When one is being in maximum-Qian (亢), one is advised to stop fire,
Ji (既） and Wei (未) are used to signify the beginning of everything,
Fu (復）and Gou (姤) are used to signify uninterrupted flow (of chi),
The degree of Moon's fullness is used to signify the level of energy,
The rise and fall of the Sun is used to signify the level of Fire.
The Master used I-Ching's signification system to illustrate the inner logic (of Neidan),
Once you have understood the logic you can forget about the signification system.
The logic is used to guide us into the area of subtle feeling and perception,
Once one can feel and perceive, the logic can also be discarded.
Those who are successful find it simple and easy,
Those who are not successful find the path complex and will be entangled inside incomprehensible concepts of the signification system.
My advice to Neidan practitioner:
When you read Can Tong Qi,
Don't (only) labour yourself inside its complex signification system!
Paul's comment: Good advice! Compare: The Diamond Sutra chapter 6: "Monks, you must always remember my Dharma-as-a-raft metaphor: Even my Dharma can be cast away, more so for those unrelated to my Dharma" - 汝等比丘,知我說法,如筏喻者,法尚應捨,何況非法。