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.
The Nine Verses are guidelines to how the key points of the body are held in Xingyi. They all pertain to the San Ti posture, but should be observed at all times, regardless of which hand or foot is forward.
Body - Bowing forward or bowing backwards, the jing cannot be strong. Leaning to the left or right are the sickness of the body posture. Erect yet leaning, leaning but erect, the posture should remain alive yet not rigid.
Shoulders - The head presses upwards. The shoulders drop downwards. The left shoulder turns, the right natually follows. The strength will arrive at the hands. This is because the shoulders are dropped.
Arms - When the left arm is extended, the right arm should gaurd the ribs. Like bent but not bent. Like straight but not straight. Curved too much they will not reach. Curved not enough, there will not be enough power.
Hands - The right hand gaurds the ribs. The left hand high as the chest. The rear hand is bent slightly. The front hand extends strongly. The two hands protecting the entire area, using only as much strength that is necessary.
Fingers - The five fingers are spread apart and shaped like hooks. The tiger's mouth is round. Like hard, like soft. The strength must arrive at the fingers and must not be forced.
Legs - The left leg is in front. The right leg supports the rear. Like straight but not straight. Like bent but not bent. Although they are both straight and bent, each move is like the chicken form.
Feet - Left foot points straight forward, leaning sideways is sick. The right foot is diagonal, the front heel faces the rear. The distance between depends on the person. The toes hook the ground to firm the root.
Tongue - The tongue is the extremity of the flesh. When it is pressed up, the qi desends. The eyes wide open, the hair stands up. The qi must sink to the Dan Tian, then the skin will be like iron and the internal organs will be strong.
Buttocks - When the buttocks are slightly tensed or "lifted," the qi will reach the four extremities. Both legs twist inwards, the buttocks press together. If the posture is too low, the shape is broken, therefore it should be raised slightly..
You must become sensitive to your surroundings at all times, seeing everywhere as if you had ten eyes.
Refers to the stepping.
Stamping down with the whole body.
The whole body drills forward.
The whole body contracts, closing space.
Legs shear with scissor stepping.
Refers to the six internal & external harmonies.
Inner and outer are one.
Appearing straight but is leaning. Appears to lean but is straight.
The hands massage the internal five elements
Jing "Lower Legs"
The place that sends the body out with sudden power that alerts the four extremities, never breaking intent.
Qi Luo "Rise and Fall"
Rise is to go out. Fall is to strike. To rise is to strike, to fall is also to strike. Rise and Fall like the overturning waves.
Jin Dui "Advance and Retreat"
Advance with low steps and retreat with high ones. If one does not understand this, it is a waste of time
Yin and Yang
Yin contains Yang. Yang contains Yin. In boxing they unite. When they come together this is e great treasure.
Wu Xing "Five Elements"
The Internal five elements must move. The external five elements must follow.
Dong Jie "Movement and Stillness"
Stillness is the original body. Movement is body in use. Stillness does not reveal it's power. Movement doesn't leave a trace. When movement and stillness have not quite issued power, this is movement and stillness.
Xu She "Empty and Full"
Emptiness is pure. Fullness is spiritual. When the pureness and spiritual are complete, one will nourish the Dan Tian and lead a long life.