Wu Chi is the Beginning, the Void from which the Universe evolved; and Tai Chi, the Grand Ultimate, was born of Wu Chi, and is the mother of Yin and Yang, the harmonious polar opposites.
Yang Cheng-fu Style Tai Chi Chuan incorporates the principles of Yin and Yang in its movements a practice. The Yin/Yang Symbol is drawn many ways, each of which has its own meaning, but for the purpose of Tai Chi Form practice, the Symbol is given as it was given to me by my masters.
There are four (4) elements to the Symbol:
The outer circle, which represents Wu Chi.
The Yin and Yang Fish Tails; Yin is full (dark), Yang is empty, (light)
The separation of Yin and Yang, heaven and earth, mind and body, with no shades of gray, with all being shades of gray.
The Fish Eyes. Yin has an un-shaded circle and Yang has a shaded circle, both are placed the respective center of the Fish Head (large portion) and represent the shades of gray which permeate all things.
For Tai Chi From practice, the Yin/Yang symbol is tilted 23.5° to the vertical, the same tilt of the earth (obliquity of the ecliptic) at the beginning the 20th Century, with the top of the Symbol representing North.
The Yin (dark, active) Fish is to the left (West) with the active head on top, symbolizing the rotation of the earth from West to East (the sun rising in the East - the direction of rotation - and sets in the West).
The Yang (light, inactive) Fish is to the left (West) in external styles, such as Hung Gar. (This is a traditional placement that originated with the original use of external meaning foreign, ie. Buddhist styles, as opposed to the "internal" Chinese, Taoist, styles, with the foreign styles being considered hard and without Chinese sophistication.)
The Yang Form commences facing South with the 36 Tain Kong Gang behind you. These are the 36 beneficial Sprits that are located in the Black Tortes stars of the North.
This is how I was taught the Form from my childhood. The first time I met Yeung Sau Chung, he asked me show him the From as I had learned it. As I began, he asked what direction I was facing; and I told him, "South."
Master Yeung was a somewhat surly man who never really smiled during the sessions I had with him, but I could see the hint of a smile when he asked me how I knew I was facing South, and I told him it was because that was the direction I was facing. He then turned to my left (my East) and asked me what direction he was facing.
I told him, "North."
He asked why he was facing north, and I told him it was because he was the Master, and the Master always faced North when he taught his student.
He frowned and asked if I thought I was his student, and I told him he was facing North, so I must be. He obviously liked the answer, because over the next few months he taught me the differences between what his father, Yang Cheng-fu taught, and what he taught.
This illustrates four important points of Yang Tai Chi.
(1) The Form always commences facing South, no matter what direction you are actually facing;
(2) The instructor always faces the student when the form commences, and is therefore facing North, no matter what direction he actually faces. Thus, his movements mirror the student.
(3) The direction of the first move from the (South facing) Wu Chi position, is Open Water Gate, and always done to the East.
(4) The direction of the last move of the Form, prior to returning to the (South facing) Wu Chi position, is Trace Eyebrows and the striking hand always strikes to the East (right). Thus the opening and closing striking moves of Form are always to the East, your right.
There are 36 northern stars (Tain Kong Gang) which are considered to be good spirits which the Tai Chi Student should have at his back to help defend against the bad influence of the 72 Earth Spirits (Di Qiu Sha). This is of course purely superstition (fear of gods) and should not be taken literally, but rather as coming from a traditional belief system that influenced every facet of Chinese life. Thus, one is protected by the Sprits of the sky (stars) above (north) against the bad or evil influences of the earth (south).
Unfortunately, some Tai Chi Chuan instructors took this literally, and taught that facing north would allow bad influences to attack one from behind, causing illness and other maladies. Because of this, they believe that performing the Form on the left side, (as the instructor does so his students can mirror him) will cause illness. Yang Cheng-fu was said to perform the Form with equal skill on both sides. His son, Yeung Sau Chung, and top student, Tung Ying Chieh taught the left side to all advanced students; and, from my personal observation, all of my Tai Chi instructors who trained under Yang Cheng-fu were equally skilled on both left and right sides.
Many wrongly attribute the origin of Yin Yang symbol to Taoism (Dao), but the symbol and concepts existed before Taoism, and coexisted with the Five Element theory. While the Yin Yang principles were adopted by Tao, they were also accepted by nearly all Chinese, regardless of religious belief. The original meaning of Yin was, dark, shady, cold, secret, mysterious, cold, and it was associated with the north side of the mountains, the shade of trees and the shaded side of a river bank. It is for this reason, I was told, that the head of the Yin Fish was on top, north. Yang, conversely, means sun, heat, clear and bright, or the South, and is to this clear and bright direction one faces at the commencement of the Form.
I've noticed that some members of the Yang family put the Yang symbol at the top, but I've never heard an explanation for that, other than it is their family name. This positioning of the Fish is most often used by Hard styles, and it may well be that Yang Cheng-fu placed the Yin at the top to distinguish Soft from Hard, not that it really matters.