There is a great deal of literature on the theory and method of Xingyi practice. Unfortunately, they are mostly in Chinese. Recently some of those source material have been translated into English. The old, Emptyflower.com site did a remarkable job highlighting some of the key points of those Xingyi Classics. Ottawakungfu.org have reproduced their work since the site went dark.
Xingyiquan was originally called Xinyi Liuhe Quan (Heart Mind Six Harmonies Boxing). This was in reference to the 3 internal and 3 external principles that are the key element to Xingyi training. The six harmonies are:
1. The hands harmonize with the feet,
2. The shoulders harmonize with the hips,
3. The elbows harmonize with the knees,
4. The heart harmonizes with the intent,
5. The intent harmonizes with the qi,
6. The qi harmonizes with the power.
In order for your jing to be effective and powerful, these principles must be followed, regardless of the style you practice.
When the hands and feet arrive together in sync, the shoulders relaxed while the hips firmed internally, and the elbows are dropped while the knees lock, then the entire body will move as one unit. Through diligent practice these movements become natural. The emotional mind will remain calm and concentrated, and the qi will flow unimpeded.
If the hands are raised but the feet are not raised, it is a waste of time. Never move the hands for no reason. The elbows do not leave the ribs, the hands do not leave the heart. The hands enter and exit openings and the body follows close behind. Wait until the opponent's intent lags, then attack. The shoulders push the elbows, the elbows push the hands.
- Dai Long Bang, 1750 A.D.
The old classics say the following eight characters must be present to make the enemy yours. Each of these characters has three important physical points which will help in aligning the body properly. These principles must be observed when practicing Xingyi, especially in the five elements.
San Ding - "three press upwards"
San Kou - "three hooks"
San Yuan - "three rounds"
San Min - "three quicks"
San Bao - "three embracings"
San Chui - "three sinkings"
San Qu - "three curves"
San Ting - "three stretches"
Three Press Upwards - the head presses upwards helping to straighten the spine, the palms press outwards and the tongue presses against the roof of the mouth.
Three Hooks - the shoulders hook forward and inwards emptying out the chest, the backs of the hands and feet must hook (the ground) and the teeth must hook (close) together.
Three Rounds - the back is rounded out, the chest rounded in and the Hu Kuo (Tiger's Mouth) is round. This is the space between the thumb and index finger.
Three Quicks - the heart (mind) must be quick like a tiger ready to pounce, the eyes must be quick like an eagle diving on it's prey and the hands must be quick, so they can strike first.
Three Embraces - the qi is embraced (held) in the Dan Tian, the heart is embraced so as you are in control no matter what happens and the ribs are embraced by the elbows, protecting them at all times.
Three Sinkings - the qi must sink, the shoulders must sink and the elbows must sink.
Three Curves - the arms must be curved, never straight, the knees must be curved and the wrists must be curved, then the energy can be used continuously by having the body naturally expand and contract.
Three Stretches - the neck must be stretched upwards, the spine must be stretched straight and the knees must stretch out like a tree creating roots.
The Eight Vital points are of first importance; they are the mother of Xingyiquan. Whether you are practicing the five Five Elements or the Twelve Animals, they must follow the Eight Vital Points.
The insides must be lifted - lift the anus, press the tongue to roof of mouth, and lift the crown point (Bai Hui) of the head.
The three hearts unite - the crown point, heart palms and heart of the sole of the feet are imagined being drawn to the center of the body.
The three intents must follow one another - Qi, strength & intention.
The Five Elements must flow smoothly - this refers to both internal and external elements in the body. The internal are the heart, liver, spleen, lungs and kidneys. The external are the tongue, eyes, mouth, nose, and ears.
The Four Extremities must move together
The heart must be at ease
The three points must be on a line The nose, front hand, and front foot.
The eyes must focus on a single point.