The Secret of Yin Yang (陰陽訣) by Yáng​ Bān​hóu​ (楊班侯)

陰陽訣

太極陰陽少人修，吞吐開合問剛柔。
正隅收放任君走，動靜變化何須愁。
生克二發隨招用，閃進全在動中求。
輕重虛實怎的是，重里現輕勿稍留。

楊班侯

"There are few people that study Tai Chi's Yin Yang. It asks for hardness and softness, to take in and send out, to open and to close.
Accepting the directions and the corners allow you to move as you wish, there is no need to worry about changes in the movement.
Producing and collecting both are used in the maneuvers of sending out and following. Both dodging and advancing can be found inside the movements.
Lightness and heaviness are within the substantial and insubstantial, do not hesitate to show lightness within heaviness."

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 Yáng Style Tàijíquán (楊氏太极拳)

Part 1:

Part 2:

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.