As yin and yang principles alternate their expressions, different qualities of chi or vital energy have been combined and represented in Fu Hsi's trigrams (kuas). By gathering trigrams, we have I Ching hexagrams.
Main and nuclear trigrams create the structure of hexagrams, and we can appreciate those combinations in the roots of Tai Chi's moves as well. It means we'll find specific parts of the body set in motion and following a specific path according to trigrams and elemental interactions.
As chi's qualities change, trigrams change too, so everyI Ching hexagram comes out from some other preceding one as well as from the nuclear one: they're part of different sequences. Similarly, Tai Chi displays forms by gathering sequenced moves. Each sequence is a form that might be related to a particular hexagram, resembling some aspect of the type of motion the sign expresses. And as we see sequences of hexagrams, each form follows a previous one too, like a continuous river flowing on and on, thus creating series of forms.
The lines of hexagrams acquire a functionality depending on the characteristics of the related energies and the position inside of the structure (see the figure below). The structure is like a growing tree, like a human figure reaching Heaven from Earth. The same principle is present in Tai Chi's moves, since all parts of the body (feet, legs, torso, arms, hands, head) must be coordinated with the mind and breath.
There's 13 moves in Tai Chi Chuan, usually divided into two groups. The first 8 ones are associated to trigrams, forming the Pa Qua, and they're called the ancient Heavenly arrangements: the Four Primary Hands associated to the 4 annual seasons (main directions), and the Four Corner Hands (in between).
The other group includes 5 steps of more extensive moves of feet and legs. These ones, as a whole, are referred to as the Five Elemental Phases of Change, as their are also known in I Ching:
As seen, all of them indicate different types of actions and orientations to make a move. For example: an resembles the waxing and waning Moon as it presses forwards, from the West (k'an, water) and looking at the left for interaction (tso ku is the step associated to water). P'eng or pulling back is associated to kun (earth), and evades incoming energy by adhering to it, although keeping the own posture (chung ting is the corresponding step for keeping balance), and it also bounces the energy back like a large inflated ball.
We may associate trigrams in several ways, by thinking on other moves, situations and purposes, and even by visualizing a sequence to create a form according to those specific interactions. Finally, the name of a form usually comes out as a combination of all those relations and associations.
A: main kuas. B: nuclear kuas. C: Earth's bigram. D: Men's bigram. E: Heaven's bigram. F: corresponding lines: 1 - 4 (foot - heart), 2 - 5 (abdomen - throat) and 3 - 6 (plexus - head).
We can see the moves above in Tai Chi's forms, for example: both an (pressing forwards) and p'eng (drawing back) in Grasp the Bird's Tail, and mainly cai (pulling down) in Needle at Sea Bottom, or lie (splitting) in Slant Flying.
Several schools teach different form styles but the following references are based on the series Da Liu explains in his book T'ai Chi Ch'uan and I Ching. A Choreography of Body and Mind (Routledge & Kegan Paul, 1981). Born in Chine, Da Liu has tought Tai Chi Chuan for more than 20 years in US. Although he started to learn in 1928 in East Chine with master Sun Lu-tang in the Sun style, he approached later the Yang style from the XIX century, after travelling to Southwest provinces. In his book, all of the forms may be considered as a sequence representing the entire motion of the fire star since sunrise until sunset, between Chin (35) and Ming (36) of I Ching.
The Beginning of the T'ai Chi Ch'uan & Chin (35)
"Stand erect, hands easily at sides, palms back. Heels are together, toes slightly apart. Sinking slightly with soft knees, shift weight onto right foot and step with left foot, toes straight forward, to the side so that feet are shoulder-width apart. Shift weight to left, and pivot on right heel to move right toes straight forward. Distribute weight evenly on both feet. Let arms rise upward to shoulder height in front; draw wrists toward shoulders, fingers slightly strengthening. Continue the circular movement, gently pressing hands down to sides again. The body rises slightly with the arms and sinks again as arms returns to sides."
Main kuas in Chin are superior Li (sun) and inferior Kun (earth, belly), symbolizing the sunrise, the fire star slowly rising higher and higher above the earth. Nuclear kuas are superior K'an (water) and inferior Ken (legs), symbolizing a standing position with a flow rising in parallel, as arms move from the belly upwards and downwards.
Crossed Hands & Ming (36)
"Shift weight to right foot, turn left toes inward, and open hands wide over head. Continue the arms down and forward and then up to cross at the wrists, chest level, palms in. The right foot pivots on toes and steps back to original straight-forward position."
Main kuas in Ming are superior Kun (earth, hidden) and inferior Li (sun, heart), symbolizing the sun going down beneath the earth, as a fire remaining hidden behind hands before the chest. Nuclear kuas are superior Chen (thunder, awakening) on Kun, and inferior K'an (abyss, ear) on Li, symbolizing the dangers of raising what is hidden as hearing the light within.
Grasp the Bird's Tail & Chien (1)
"The left hand reaches up, palm inward, to hold the bird's neck; the right hand moves down as though smoothing the long plumage of the bird's tail. Shift weight to left leg, pivoting on right heel, turn to right. Simultaneously, the right arm rises, elbow bent, with hand at shoulder height; the left arm rises and crosses body at waist level, palm up. The palms are approximately toward each other. Shift weight toward onto right foot and pivot on left toes to turn slightly to the left. Step left with left foot, straight forward and a little left. Simultaneously, the weight shifts forward on left foot, the left hand moves up to chin level, palm facing in; the right arm returns to right side, and as the body turns to left the right toes moves slightly inwards."
The form derives from Chien The Creative (1). The hands seem to grasp a bird's tail and head at the belly and chin of the practitioner. The bird is the dragon referred to in the nuclear lines of the main kuas (chien): the "field" (2°) is the abdomen, and the "heaven" (5°) is the chin.
Single Whip & Ko (49)
"The left hand resembles a single whip. The weight shifts to left leg, which slightly straigthens arms, while the right toes are lifted. Pivoting on the right heel, turn body 135°, arms still extended and moving with the body. Shift weight to right foot and pivot on left toes, turning body to face left. Simultaneously left arm moves across body at waist, palm up; right arm extends to right side of the body, fingers pinched, hand at shoulder height. The elbow points to the floor. With weight on right leg, pivot on left toes to turn left, facing forward, opening arms to front. Left foot takes a wide step forward and to the left. The weight moves to the left foot, the left hand turns palm over as it moves up to press with fingertips at throat height as the right toes move inward, pivoting on heel."
After 4 seasonal moves (Primary Hands), Earth has completed a revolution around the Sun. The form comes out of hexagram Ko Revolution (49) and follows the previous one (Grasp Bird's Tail, The Creative). Main kuas in Ko are superior Tui (metal, right) and inferior Li (fire, horse), symbolizing a wide and wild expanding move. Nuclear ones are superior Chien (turning) on Tui, and inferior Sun (wind, left) on Li, symbolizing a gentle 135° flowing motion in the opposite direction, like a soft wind of light.
Tai Chi is considered as an "internal" martial art focused on developing chi (vital energy), by working in the dynamic relationship between yin and yang flows, reaching effortless action (wu wei), expressing naturalness and experiencing a no-mind state (wuxin).
Body and breath are coordinated in physical moves of deep relaxation rather than muscular tension, by attending inwards and slowing the mind, as the pelvis moves and rotates in circles like the hub of a wheel, and arms and feet move as spokes.
The art requires accurate footwork and legwork forms, routines, and "push hands" exercises. But only when the body is balanced, aligned with gravity, it's possible to fully relax downwards through the legs into the earth. Footwork develops five skills named wubu (wu = five + bu = step), which are quite relevant for body motion. The body follows the steps to move, and only when the body can move to the right position (distance and angle), then the hand skills can work well.
There's six qualities of energies with a functional use: adhering, sticking, neutralizing, seizing, enticing, and issuing, always inside of Tai Chi's endles quest for conservation of the energy. The practitioners quickly learn how to read other people's moves, moments and intensities, and know what to do to keep themselves protected while looking for an opening. By constantly adapting the moves to suit the environment and the person they meet, they develop the ability to adapt to ever-changing circumstances, since life is constantly changing at an ever-increasing tempo.
The practitioners also learn to put the full weight of their bodies behind every part of every move. Even if they are moving only one inch, they still have full power there. That power comes from connecting all the muscles together, not by isolating and training specific parts. As we see in I Ching hexagrams, including the dynamic of all of the forces inside of a situation, is everything.
In the following video, master Yijiao Hong performs 24 forms in Yang style as a single sequence of approximately 10', edited in a way we can appreciate the moves from two simultaneous points of view.