To acquire maximum power from the movements it is necessary to employ sum qua. The qua are the four emptinesses - two in the armpits and two where the leg joins the torso, i.e. the crease of the panty line. In this case we are referring to the qua of the leg crease. Sum means to sink. So it is necessary to sink into the qua of the lower body in order to align one's torso in the direction of the force. This enables the entire body to push, press, or punch forward at the same time. Without sum qua there is only partial power which is easily deflected. Sum qua is also the key to neutralizing an opponent's force. Sinking into the qua, i.e. folding the torso, makes you so light that your opponent has nothing to push against - like trying to strike the air.
More than Chi Gung vs Nei Gung
Chi Gung has to do with breath and energy, while Nei Gung has to do with rooting and power. Chi Gung tends to be related to health, while Nei Gung tends to be connected to the martial. Ideally the Tai Chi practioner incorporates both into their practise.
Although both are important, more important is the purification of the Soul of Ego accumulations - cleaning the Mirror of the Mind of the cultural conditioning of a lifetime - polishing our luminous Egg of debris so that it shines brightly into the Universe. This is what Master Ni was all about. This is what was unique about his classes. This is what drew us to him. This is what we try to transmit.