An essay on the Taiji Quan Lun (attributed to its legendary creator Zhang San Feng 张三丰)
When these requirements are fulfilled, no matter how one moves: the superior position and advantage is gained. If superiority is not acquired it's because one is disordered.
One of the benefits of this internal work for a two person exchange: an optimal reaction and response
If the all body is animated with Qi, i.e. the nerve impulse evenly fills the body, then one will surely get the advantage as he will have the optimal move, optimal timing and optimal reflex. If the advantage cannot be gained, it's because one of the previous points was not respected: body and mind are not unified, body parts are not unified, and thus one is disorganized. The text then mentions that the cause of the error is either in the legs and waist, which often relates to the most common errors of posture or in too much intention in the releasing extremities.