The Sky (Qian) and the Earth (Qun) trigrams signify the opening door of I-Ching
As Father and Mother of all hexagrams
The Water (Kan) and the Fire (Li) trigrams act as a vast empty container
Acting like the hub of a wheel at the hollowed center
Follow the symbolism of these four male and female trigrams
Acting as wind box to power the whole process
Paul's comment: Most Taoist yoga practitioners view Can Tong Qi (周易參同契) as a definitive text of their practice. It often times came across as rather mysterious for readers. Scholars of all ages, and I would include unseasoned practitioners here, failed to understand them. Since the trigrams and hexagrams, with their historical interpretations, never fail to cover aspects of human affairs as self-regulating systems, and Taoist yoga (meditation or neidan) is one advanced practice based on the principles of self-regulating systems, it will be interesting to learn how the esteemed author (Wei BoYang 魏伯陽) interpreted this profound Taoist practice using I-Ching.
Having said that, for those who are not seasoned practitioners and are interested to learn the techniques, I would suggest the texts Hui Ming Jing (慧命經) and Taoist Yoga (性命法訣明指） instead.