The attacking movements of the Fen Jiao and Deng Jiao kicks in Yang Style Taijiquan without exception require standing straight on one leg, with a stable lower frame. To achieve a stable lower frame you must pay attention to the following: the straight standing leg must not be stiff, the torso must not draw up, and the qi should not float up; the lower form should possess a downward-sinking strength (下沉的力量). The classical taijiquan requirements, “an intangible and lively energy lifts the crown of the head,” “loosen the waist and qua,” “contain the chest and pull up the back,” “sink the qi to the dantian,” and “sink the shoulders and drop the elbows,” should all be implemented in these movements.
The movement of the foot in both Fen Jiao and Deng Jiao is to lift the knee and employ the springy jin (彈勁) to kick out in an instant. It is not a use of the waist/kua jin (腰跨的勁), nor a use of strength from the thigh (大腿根也不用力). If you were to employ strength (用力) from the waist/kua, then it would change into a thrusting kick (chuai 踹 ), which is the foot method of other boxing styles. In contrast, according to the kicking method of Yang Style taijiquan, the use of strength from the waist/kua will cause your body to be uncentered, and the strength to dissipate and disintegrate. The characteristics of Yang Style taijiquan’s kicking method is completely in harmony with the idea expressed in the classics: “it is rooted in the feet, issued by the legs, governed by the waist, and expressed in the fingers. From the feet, to the legs, then to the waist, always there must be complete integration into one qi.” The foot that is kicking out is rooted in the other [standing] foot. The foot, waist, and kua link together to create a stable base. This is foundation for the other, kicking foot. Carefully ponder this method of using jin, and you will come to intuitively understand it in your mind.
—Yang Zhenji, 杨澄甫式太极拳 Yang Chengfu Shi Taijiquan, page 88.
（３）分、蹬脚等脚的动作是提膝用脚的弹劲一瞬间踢出，不是用腰胯的劲，大腿也不用力。如果腰胯用力，就会变成踹，是其他拳派的的脚法。按杨式太极拳的 踢法用腰胯的力量反而使身不中正，力量分散不整。杨式太极拳踢法的特点与古典“其根在脚、发于腿、主宰于腰，形于手指，由腿而腿而腰，总须完整一气”的说 法是相融的。踢出的脚的根在另一脚，脚、腰、胯相连坐稳，是踢出的另一脚的基础。细心揣摩这种用劲的方法，自然能体会于心。
Translation: Louis Swaim