On Joining and Supporting of Yin and Yang
Refining the Shape is not beyond Yin and Yang. How will one practice if Yin and Yang are not clear? The Du Channel in Taoist classics goes along the middle of the back and commands all Yang Channels. Ren Channel goes along the front of the body (Fu, literally: belly) and commands all Yin Channels. That is why back is considered Yang and front of the body is considered Yin. Both channels meet at Hui Yin at the bottom, and at the gums at the top. South and north, they are opposite like midnight and noon. Or like Kan trigram which resides at the center of north, and Li trigram that resides at the center of south, not easy to define.
Bending forward posture is a Yin posture, but it joins Yang Qi and is beneficial to Du Channel. It leads Qi of all Yang channels, and returns completely to the front of the top.
Bending backward posture is a Yang posture, but it joins Yin Qi and is beneficial to Ren Channel. It leads Qi of all Yin channels, and returns completely to the back of the top.
On Moving Qi
The point of contact (Luo Dian) is hard and solid. Fierce and brave, irresistible. (It) relies on Qi of the whole body, but yet (Qi must) concentrate in one place. Nevertheless, it can be used without loss of Qi. It is harmful if Qi is lost or pulled. It means that one does not know the method of Moving Qi (Guo Qi). All Qi of the body has its beginning in the Gate of Life, which is the source of Qi. Qi is manifested in four ends, which fill with Qi. (Qi) flows along (its) paths. Generally (speaking) Qi must not be sluggish nor pulled and only then can flow beneficially (for health), be nimble and unfathomable.
Hence upper Qi is in the bottom, so the bottom (must) not be pulled (e.g. stopped) if (one) wants to move downwards.
Lower Qi is on the top, so the top must not be sluggished if (one) wants to move upwards.
Front Qi is in the back, so the front will naturally enter if (one) smoothens the back.
Back Qi is in the front, so the back will naturally go away if (one) regulates the front.
Left Qi is in the right, so pay attention to the right.
Right Qi is in the left, so pay attention to the left.
For instance, in straight strike with palm, Qi should flow forward. If the other hand is not pulled back by the elbow, (then) Qi is not allowed to flow forward from the back. In upward strike, if the other hand does not insert (e.g. move downward) and shoulder does not drop, (then) Qi is not allowed to flow upward from ribs.
In separating techniques, if the chest does not open, then Qi is not allowed to flow backward. In embracing techniques, if the chest does not open (should be: close), then Qi is not allowed to wrap the front. While rising (Qi), (one) must hook the foot. While falling (Luo) (one) must draw back the crown of the head. Qi of left hand is in the right leg, Qi of right hand is in the left leg. In bending forward posture, tumbling posture and forward exploring posture, lift the heel of the rear foot. In dropping, sink arms. In lifting posture, turn over the feet. In tumbling, do not lift up feet (because you) may hit the ground with (your) head. In kicking do not straighten the leg, (but) consider drawing it back. Expand and strengthen it. All postures are like that.
To summarize it - during contact Qi moves to one place. (However) Qi does not come from one place. Its paths will be smooth only if (you) dredge its source and clear its course. (Your will) suffer from being sluggish (stagnant) or pulled Qi (and you will) not progress unless (you) advance gradually and dig in at every step,