The Secret of Insubstantial and Substantial (虛實訣) by Yáng​ Bān​hóu​ (楊班侯)

虛實訣

虛虛實實神會中，虛實實虛手行功。
練拳不諳虛實理，枉費功夫終無成。
虛守實發掌中竅，中實不發藝難精。
虛實自有虛實在，實實虛虛攻不空。

楊班侯

"What is substantial and what is insubstantial are gathered within the spirit, the differentiation of the substantial and insubstantial is the achievement of the hand.
To practice boxing without understanding the essence of the difference between substantial and insubstantial is to waste one's effort to achieve nothing at the end.
Defend with the insubstantial, attack with the substantial, the trick is within the palms. If one stays substantial in the center, there is  no send-out skill and the energy is distressed.
Insubstantial and substantial have a concrete reason, with the differentiation of insubstantial and substantial, attacking will not be in vain."

Based on the translation by Yang Jwing Ming (楊俊敏) presented in the book Tai Chi Secrets of the Yang Style (太極拳楊氏先哲秘要)

Fu Zhongwen (傅鐘文) performing Tai Chi Sword (太極劍), Spear (太極槍) and Fajin (發勁).

The Song of True Meaning of Tàijíquán (太極拳真義訣) by Lǐ​ Dàozi​ (李道子)

One of the difficulties one may encounter when looking at the texts presented as important for the practice is their cryptic language. It can be understood that some of the inner experiences can only be passed on in poetic form. It is also understood that early texts come from oral traditions tend to come in "songs" or texts that are easier to memorize.
For example, this text has the suggestive name of "The Song of True Meaning of Tai Chi Chuan". It certainly attracts attention also because it is reported by two important authors like Yang Jwing Ming (楊俊敏) and Jou Tsung Hwa (周宗樺). So, definetely something that the serious practicant should pay attention to.
In the other hand, beyond the evocations it brings, it is difficult to have an indication of how it should influence our practices. At least, if it is not commented by the practice with a recognized Master.

太極拳真義訣

無形無象，全身透空。
忘物自然，西山懸磬。
虎吼猿鳴，泉清水靜。
翻江鬧海，盡性立命。

李道子

"No shape, no shadow.
The entire body is transparent as air.
Forget your surroundings and be natural.
Like a stone chime suspended from the West Mountain.
Tigers roaring, monkeys screeching.
Clear fountain, peaceful water.
Turbulent river, stormy ocean.
Develop your whole body and mind to their full extent."

From the book The Tao of Tai Chi Chuan" (太極拳之道), by Jou Tsung Hwa (周宗樺) and "Yang Style Tai Chi Chuan" by Yang Jwing Ming (楊俊敏) based on the translations proposed in both books.

Journal of Asian Martial Arts

The Journal of Asian Martial Arts is probably the only magazine that I could add within my references of study, along with the Pa Kua Journal. Far from the screaming pictures of martial arts magazines, it has a serious and scholar style.
There are really two types of articles. The first one is the kind that link martial arts with its context, historical, artistic, cultural, religious. These articles imply field research and academic insight. They are by far the most interesting, as they throw some light on aspects that would otherwise remain unperceived.
The second kind of article is the more common kind centered around techniques. They have the merit of being sober and presenting little known schools or aspects. In the other hand, they do not differ much from the more common technical manual.
One of the sections of the magazine is of particular interest: the book reviews are well done and do present very relevant critiques.

Stretch (挺) from the Xingyi Eight Characters Secret (形意八字訣)

挺

有三挺，即
一。頸項挺，則頭部正直，精氣貫頂。
二。脊骨腰挺，則力達四稍，氣鼓全身。
三。膝蓋挺，則氣恬神怡，如樹生根。
是謂之三挺。

李存義

"Stretch. The three things that must be stretched:
a) When the neck stretches, the head is straight and the jing and chi can reach the top.
b) When the spine is stretched, the force reaches the four limbs and the chi permeates the whole body.
c) When the knees stretch (out), the qi is calm and the spirit is in harmony, like a tree with strong roots.
These are the three stretches."

From the book Xingyi Connected Fist (形意連環拳), by Lǐ Cúnyì (李存義), and from Xingyi Mother Fists (形意母拳) by Jiāng Róngqiáo (薑容樵) based on the translations by Joseph Crandall

Bend (曲) from the Xingyi Eight Characters Secret (形意八字訣)

曲

有三曲，即
一。兩臂要曲弓如半月，則力富。
二。兩膝宜曲，彎如半月，則力厚。
三。手腕宜曲，曲如半月，則力湊。
皆取其伸縮自如，用勁不斷之意。
是謂之三曲。

李存義

"Bend. The three things that one must be bent:
a) Both arms ought to be bent, like a half-moon, the strength is abundant.
b) Both knees ought to be bent, like a half-moon, the strength is deep.
c) Both wrists ought to be bent, like a half-moon, the strength comes together.
All of them gets its natural extension and contraction, the strength of the intention unbroken.
These are the three things that must be bent."

From the book Xingyi Connected Fist (形意連環拳), by Lǐ Cúnyì (李存義), and from Xingyi Mother Fists (形意母拳) by Jiāng Róngqiáo (薑容樵) based on the translations by Joseph Crandall

Let Fall (垂) from the Xingyi Eight Characters Secret (形意八字訣)

垂

有三垂，即
一。氣垂則氣降丹田，身穩如山。
二。兩肩下垂，則臂長而活，肩催肘前。
三。兩肘下垂，則兩肱自圓，能固兩脅。
是謂之三垂。

李存義

"Let Fall. The three things that one must let fall:
a) When the qi is suspended, the qi descends to the dantian, and the body is stable as a mountain.
b) When both shoulders hang down, the arms are long and lively, and the shoulders push the elbows to the front.
c) When both elbows hang down, then both arms are naturally round, and can solidify the ribs.
These are the three things that one must let fall."
From the book Xingyi Connected Fist (形意連環拳), by Lǐ Cúnyì (李存義), and from Xingyi Mother Fists (形意母拳) by Jiāng Róngqiáo (薑容樵) based on the translations by Joseph Crandall

Embrace (抱) from the Xingyi Eight Characters Secret (形意八字訣)

抱

有三抱，即
一。丹田要抱，氣不外散，擊敵必準。
二。心氣要抱，遇敵有主，臨變不變。
三。兩脅要抱，出入不亂，遇敵無險。
是謂之三抱。

李存義

"Embrace. The three things that must embrace are:
a) The dantian must be embraced, the qi is not let out. Stricking the opponent needs precision.
b) The emotions must be embraced. When meeting the opponent, have self-control. Don't change when facing change.
c) Both ribs must be embraced. Then leaving and entering is not confused and meeting the opponent is not dangerous
These are the three things that must be embraced."
From the book Xingyi Connected Fist (形意連環拳), by Lǐ Cúnyì (李存義), and from Xingyi Mother Fists (形意母拳) by Jiāng Róngqiáo (薑容樵) based on the translations by Joseph Crandall

Agile (敏) from the Xingyi Eight Characters Secret (形意八字訣)

敏

有三敏，即
一。心要敏，如怒狸攫鼠，則能隨機應變。
二。眼要敏，如饑鷹之捉兔，能預視察機宜。
三。手要敏，如捕羊之餓虎，能先發制人。
是謂之三敏。

李存義

"Agile. The three things that must be agile are:
a) The heart must be agile like an angry cat catching a mouse. Then you will be able to change according to the situation.
b) The eyes must be agile, like a hungry eagle catching a rabbit. The you will be able to examine in advance what to do.
c) The hands must be agile, like a hungry tiger snatching a goat. Then they can issue first to control the opponent
These are the three things that must be agile."

From the book Xingyi Connected Fist (形意連環拳), by Lǐ Cúnyì (李存義), and from Xingyi Mother Fists (形意母拳) by Jiāng Róngqiáo (薑容樵) based on the translations by Joseph Crandall

Round (圓) from the Xingyi Eight Characters Secret (形意八字訣)

圓

有三圓，即
一。脊背要圓，其力催身，則尾閭中正，精神貫頂。
二。前胸要圓，兩肘力全，心窩微收，呼吸通順。
三。虎口要圓，勇猛外宣，則手有裹抱力。
是謂之三圓。

李存義

"Round. The three things that must be round are:
a) The back should be round. Its strenght breaks the opponent's body, and the sacrum will be centered and upright.The spirit of vitality will permeate to the top of the head,
b) The chest must be round. Then both elbow will be completely strong, the heart cavity slightly contracts, breathing is unhindered and smooth.
c) The tiger's mouth must be round. Then bravery and fierceness will be proclaimed. Then the hands will be able to embrace.
These are the three things that must be round."

From the book Xingyi Connected Fist (形意連環拳), by Lǐ Cúnyì (李存義), and from Xingyi Mother Fists (形意母拳) by Jiāng Róngqiáo (薑容樵) based on the translations by Joseph Crandall