Bend the Legs, Tread Through the Mud; the Center of the Foot is Empty (曲腿蹚泥，足心涵空) from The Real Theory of Baguazhang (八卦掌真理篇) by Jiāng Róngqiáo (薑容樵)

曲腿蹚泥，足心涵空

曲腿就是在走轉時兩腿作適度之彎曲，身體往下坐，使力量貫注兩腿。蹚泥是兩腳前進不要過高，如蹚泥之狀。足心涵空是使腳掌和腳跟同時平落地面，五指抓地，這樣腳心就涵空了。

薑容樵

“'Bend the legs' means that when stepping or rotating, both legs must be slightly bent and the body must assume a sitting down posture, enabling all the power to be concentrated in the legs. 'Tread through the mud' means that when both legs advance, they must not be too high, the feet look as if dragging in the mud. 'The center of the foot is empty' means that the sole of the foot and the heel of the foot fall flatly to the ground simultaneously, the five toes grip the ground, thus the center of the foot becomes hollow."

From the book Bāguàzhăng Practice Method (八卦掌練習法), by Jiāng Róngqiáo (薑容樵), based on the translation by Joseph Crandall

Féng​ Zhèng​bǎo​ (馮正寳) demonstrating Xíngyìquán (形意拳) Training

The Explanation of Taiji’s Understanding Jin (太極懂勁解 ) by Yáng​ Bān​hóu​ (楊班侯)

太極懂勁解

自己懂勁，接及神明，為之文成，而後採戰。身中之陰，七十有二，無時不然。陽得其陰，水火既濟，乾坤交泰，性命葆真矣!
於人懂勁，視聽之際，遇而變化，自得曲誠之妙。形，著明於不勞，運動知覺也。功至此，可為攸往鹹宜，無須有心之運用耳。

楊班侯

"With Understanding Jin applied to oneself, one reaches enlightment, learning is accomplished and that it can be applied to battle. Hidden within the body, seventy are two, there is no time. Yang is suitable to Yin, water and fire will support each other, the universe will revolve in harmony, life is truly preserved.
With Understanding Jin applied to the opponent, at the moment one looks and listens he sees the opportunities and the changes, and subtly adapt to the curve and the straight. With this level of achievement one can move forward unified and suitably, without the need to put the mind to use."

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)

Excerpt from On Harmony (雍也) in the Analects (論語)Trecho do Sobre a Harmonia (雍也) dos Analetos (論語)

「知者樂水，仁者樂山；知者動，仁者靜；知者樂，仁者壽。」

孔子

"'The wise find pleasure in water; the virtuous find pleasure in hills. The wise are active; the virtuous are tranquil. The wise are joyful; the virtuous are long-lived'"

Unless noted otherwise, all quotations of classical texts and accompanying translations come from the Chinese Text Project

"'O sábio aprecia as águas, o homem virtuoso aprecia as montanhas. O sábio é ativo, o homem virtuoso tranqüilo. O sábio é alegre, o homem virtuoso vive longos anos.'"

Traduzido à partir da tradução para o inglês do Chinese Text Project

The Four Antennas (四) from The Essentials of Yue Fei's Boxing Art (附岳武穆形意拳術要論)

第四章　四、要論

試于論身論氣之外，而進論乎梢者焉，夫梢者身之余緒也，言身者初不及此，言氣者亦所罕論，捶以內而發外，氣由身而達梢，故氣之用不本諸身，則虛而不實，不形諸梢，則實而仍虛，梢亦焉可不講，然此特身之梢耳，而猶未及乎氣之梢也，四梢維何，發其一也，夫發之所系，不列于五行，無關于四體，似不足論矣，然發為血之梢，血為氣之海，縱不必本諸發以論氣，要不能離乎血而生氣，不離乎血，即得不兼及乎發，發欲冲冠，血梢足矣，抑舌為肉梢，而肉為氣囊，氣不能形諸肉之梢，即無以充其氣之量，故必舌欲催齒，而后肉梢足矣，至于骨梢者齒也，筋梢者指甲也，氣生于骨而聯于筋，不及乎齒，即未及乎筋之梢，而欲足手爾者，要非齒欲斷筋，甲欲透骨，不能也，果能如此則四梢足矣，四梢足而氣亦自足矣，豈復有虛而不實，實而仍虛者乎。

岳飛

"Examining beyond the body and the energy, there are then the “antennas”. The antennas are the leftover parts of the body. They are not addressed initially when discussing the body, and hardly dealt with at all when discussing the energy. A punch expresses outward from inside, and energy goes from the body into the antennas. Thus if the use of energy does not come from the whole body, it is empty rather than authentic, and if it does not reveal itself in the antennas, then even if it is there, it is still empty. The antennas must be given attention, although they are specifically antennas of the body, not purely antennas of energy.
What are the four antennas? Firstly, there is the head hair. It has nothing to do with the organs or limbs, and thus would appear to be irrelevant, but the hair is the antenna for the blood, and the blood is the sea which the energy swims in. Although it is not necessary to consider the hair when talking about the energy, we cannot talk about the energy without considering the blood. Since we have to consider the blood, how can we not at the same time consider the hair? The hair should be pricking up under the cap, which indicates the blood is in a state of sufficiency.
The tongue is then the antenna for the muscles. The muscles are a sack for holding the energy. If the energy is not able to reveal itself in the tongue, then the energy is not at full capacity. Therefore the tongue should be pressing up behind the teeth, indicating the muscles are in a state of sufficiency.
The teeth are the antenna for the bones, and the nails are the antenna for the sinews. Energy is born in the bones and unites with the sinews, and so if it is not apparent in the teeth, it will also not show up in the nails. For it to display sufficiency in this way, the key is that the teeth will determine the state of the sinews and the nails will disclose the condition of the bones.
Once they are thus, the four antennas are in a state of sufficiency, and that being so, naturally the energy is too. It is now not the case that it is empty rather than authentic, or that even if it is there, it is still empty.

Excerpt from the translation by Paul Brennan of "The Art of Xingyi Boxing" (形意拳術) by Lǐ Jiànqiū​(李劍秋), to be found in the Brennan Translation blog.