The Entering and Support of Yin and Yang (陰陽入扶論) from the Chang Family Comprehensive Theory of Martial Arts (萇氏武技全書)

陰陽入扶論

練形不外陰陽，陰陽不明從何練起，督脈統領諸陽經，任脈統領諸陰經，故背為陽，腹為陰，二經下交會陰，上會齦交。
俯式為陰勢，卻是入陽氣，益督脈，領諸陽經之氣，盡歸於上之前也；仰式為陽式，卻是入陰氣，益任脈，領諸陰經之氣，盡歸於上之後也。

萇乃周

"Training the physical form is nothing more than yin and yang. If we fail to understand yin and yang, how would one know where to begin our training? The governing vessel (du mai) commands all of the yang channels. The controlling vessel (ren mai) commands all of the yin channels. This is the reason why the back is yang, and the belly is yin. The two channels intersect below at the Perineum (hui yin, CV-1) and above at the Gum Intersection (duan jiao, GV-28).
To look down is a Yin posture, it allows the Yang chi to enter. It benefits the governing vessel. It concentrates the chi of all yang channels, which finishes at the upper front aspect of the body. To look up is a Yang posture and allows the Yin chi to enter. It benefits the controlling vessel, leads the chi of all Yin channels, which finishes at the upper rear aspect of the body."

From the book Chang Family Comprehensive Theory of Martial Arts (萇氏武技全書), by Cháng Nǎizhōu (萇乃周), based on the translations by Douglas Wile in "T'Ai Chi's Ancestors: The Making of an Internal Art" and Marnix Wells in "Scholar Boxer: Chang Naizhou's Theory Of Internal Martial Arts And The Evolution of Tàijíquán"

Large and Small Postures (大小勢) from the Chang Family Comprehensive Theory of Martial Arts (萇氏武技全書)

大小勢

合勢不嫌其小，欲氣合得足也；開勢不嫌其大，欲力發得出也。非徒長身為大，屈身為小。
力是自然之力，故初學必以不著力為是。

萇乃周

"In closing postures one should not mind it being small, they need sufficient unification of qi. In opening postures do one should not mind it being large; they need to release the force. It is not just a matter of expanding the body to make it great or contracting it to make it small.
Power is spontaneous power, for beginners the use of force is necessarily not correct."

From the book Chang Family Comprehensive Theory of Martial Arts (萇氏武技全書), by Cháng Nǎizhōu (萇乃周), based on the translations by Douglas Wile in "T'Ai Chi's Ancestors: The Making of an Internal Art" and Marnix Wells in "Scholar Boxer: Chang Naizhou's Theory Of Internal Martial Arts And The Evolution of Tàijíquán"

Borrowing Qi (借行氣) from the Chang Family Comprehensive Theory of Martial Arts (萇氏武技全書)

借行氣

借行氣者，借人之氣方行而打之也。蓋彼之勢既發，已近我身，尚未落點，我即趁此機會，發我之勢，彼欲退不能，欲攔不及，再無不妙之理。若稍前，則彼尚可退回；稍後，則我已吃虧。所謂後發先至者，此也。如此，則迎機赴節，隨時得宜，不患技之不高矣。諸家所謂將計就計，借力使力，不外此訣。所謂驚戰計，正於此時用之。

萇乃周

"« Borrowing Qi » refers to borrowing the opponent’s Qi to strike the opponent. When the opponent’s strike is just about to land on my body, I take advantage of this to develop my posture, leaving the opponent no opportunity to retreat or to block. There is no mysterious principle. If I make my move prematurely, this allows the opponent time to retreat; if I make my move too late, then we have already been beaten. This is called of “strike second but land first”. In this way, seize opportunities and go into action. If you act at the right time, you need never worry about your lack of skill. This is what is meant by beating somebody at its own game and borrowing strength to apply strength. This is what is called by striking by stratagem at the right opportunity."

From the book Chang Family Comprehensive Theory of Martial Arts (萇氏武技全書), by Cháng Nǎizhōu (萇乃周), based on the translations by Douglas Wile in "T'Ai Chi's Ancestors: The Making of an Internal Art" and Marnix Wells in "Scholar Boxer: Chang Naizhou's Theory Of Internal Martial Arts And The Evolution of Tàijíquán"