Seeking calmness in the movements (動中求靜) from The Ten Important Points of Tai Chi Chuan (太極拳十要)

動中求靜

外家拳術，以跳躑為能，用盡氣力，故練習之后，無不喘氣者。太極拳以靜御動，雖動尤靜，故練架子愈慢愈好。慢則呼吸深長，氣沉丹田，自無血脈僨張之弊。學者細心休會，庶可得其意焉。

楊澄甫

"The fist techniques of the external styles grant expertise in jumping and walking, use the Qi and Li until it is exhausted. Therefore, right after practice, everybody will be panting. Taijiquan uses the calmness to control the movements, though moving remain calm. Therefore, when practicing postures, the slower the better. When slow, then the breathing is deep and long, the Qi is sunk to Dan Tian. Consequently, there is no harmful problem like the blood vessels' expansion. The practitioners should ponder and comprehend it carefully, then can (they) obtain its meaning."

By Yáng Chéngfǔ (楊澄甫), based on the translation by 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) and various translations on the internet