The explanation of Taiji’s Lower Level Martial Aspects (太極下乘武事解) by Yáng​ Bān​hóu​ (楊班侯)

太極下乘武事解

太極之武事，外操柔軟，內含堅剛。而求柔軟之於外，久而久之，自得內之堅剛。非有心之堅剛，實有心之柔軟也。所難者，內要含蓄堅剛而不施，外終柔軟而迎敵。以柔軟而應堅剛，使堅剛盡化無有矣。

"In the martial aspects of Taijiquan, the exterior steers soft and gentle and the interior keeps hard and strong. Over time, as softness is sought externally, one’s interior become hard and strong. One does not looks deliberately for hardness, one rather sincerely seeks softness. The difficulty is that the inside is required to keep strong and hard and not apply it, while the outside keeps gentle and soft when one meets an opponent. One uses softness to respond to strength and hardness in order to cause this strength and hardness to dissolve into nothingness."

其功何以得乎？要非粘黏連隨已成，自得運動知覺，方為懂勁。而後神而明之，化境極矣。夫四兩撥千斤之妙，功不及化境，將何以能？是所謂懂粘連，得其视聽輕靈之巧耳。

楊班侯

"How is this achieved? The key is to accomplish the training of attaching, adhering, connecting and following, obtain awareness and sensitivity of movement, then reach the Understanding Jin. At this point the spirit will clear and the conditions for a major transformation is reached. If the wonder of using four ounces to repel a thousand pounds and the conditions for this major transformation is still not yet reached, then what can one do? Apply what is called “understanding attach and adhere”, use the senses to skillfully find opportunities."

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)