‘clearly distinguish between yin and yang’, a classics’ commentary

Chia has other elements. He teaches not just ching gung (lower dan tien trainning), also chi and shen kung. Here the focus will be on lower dan tien.
Lower dan tien is center of phisical level. Hence the primary element of its activation should be phisical movement as to prevent ungroundedness.

Essential elements of martial tai chi:
Dan tien rotations (‘developing multidimensional center’)
Fa jing (‘cannon fist’, hard fa jing)
Silk reeling (skillfull use of ‘stretching, twisting and relaxing’)

classic say:
Clearly distinguish between yin and yang.

1. … of each bodypart (each joint is either yin or yang…)
2. … between left and right (if left wrist is yin, right is yang)
3. … between upper and lower (this demands futher explaination)

There are ten levels of yin/yang distribution
1. 1 yang, 9 yin
( external hand movement, legs drag along)
(large frame level)
2. 2 yang, 8 yin
3. 3 yang, 7 yin
4. 4 yang, 6 yin
5. 5 yang, 5 yin
(still external hand movement, but now powered by dan tien and silk reeling)
6. 6 yang, 4 yin
7. 7 yang, 3 yin
8. 8 yang, 2 yin
9. 9 yang, 1 yin
(full utilization of dan tien roation, movement od dan tien (and torso(and legs)) replace movement of arms)
(small frame level)
10. tai chi
(multidimensional center is fully operational, and now includes also all upper center, the whole body; and the eath center bellow)

Upper is yang. Lower is yin.
Emty the upper, fill the lower.

1. what is the purpuse of large frames
they are only instructional tools. Each hand and led movement is representative of a dan tien roation/movement. When such move is activated, the external movement is not necessary anymore.
Similar interpretation is for locked joint, as being pertinent to left and right kwa openings sequence.
2. should one train the small frame if this is the highest level
no, unless one is initiated in the intricacies of internal principles other way.
A way of learning the small frame for beginner is to practice tai chi form without hands (hands hanging down, or on dan tien or crossed across chest), or solo pracitice of one hand stick hands routines, again ‘without’ hands.
3. on getting lower
small frame is being low and small hand and leg transitions.
Large frame is being higher and larger hand and leg movement.
It is not healthy to go low and practice on the large frame level.
4. dan tien moves or legs initiate the move
dan tien initiates the moves. Yet this may be biased into developing internal strenght of hands.
If grounding into earth is fully achieved, then dan tien move twists into eart and rebound upward into upper limbs.
The rebound from earth then initates the great power of upper limbs.
This explains how legs move the body.

Courticy of Erle Moontagu.
Hands low practice is from PietroS of Kumar fame
Legs should move the hands is from Hatsumi scrolls.
Fox way listing of levels is from Chen Quanzhong book. The products of the grandmaster that they sell in us is good.