The emphasis is on the hip movement whether front or back.
The difficulty is to maintain the position without shifting the centre.
To analyse and understand the above situation is to do with
movement and not with a stationary posture.
Advancing and retreating by turning sideways in line with the
shoulders, one is capable of turning like a millstone, fast or slow,
as if whirling like a dragon in the clouds or sensing the approach
of a fierce tiger.
From this, one can learn the usage of the movement of
the upper torso.
Through long practice, such movement will become natural."
- Yang Family Old Manual, The Coil Incense Kung
"Silk reeling (pinyin chánsīgōng, Wade-Giles ch'an2 ssu1 kung1 纏絲功), also called "Winding Silk Power" (chansijing) (纏絲勁), as well as "Foundational Training"(jibengong), refers to a set of neigong exercises frequently used by the Chen style, Wu style and some other styles of T'ai Chi Ch'uan. The name derives from the metaphorical principle of "reeling the silk from a silk worm's cocoon". In order to draw out the silk successfully the action must be smooth and consistent without jerking or changing direction sharply. Too fast, the silk breaks, too slow, it sticks to itself and becomes tangled. Hence, the silk reeling movements are continuous, cyclic patterns performed at constant speed with the "light touch" of drawing silk.
In common with all Qigong exercises, the patterns are performed in a concentrated, meditative state with an emphasis on relaxation. However, rather than being isolated exercises purely for health benefits, the focus is on strengthening and training the whole body coordination (nei jin) and grounded body alignment that is used in the Tai Chi form and pushing hands. Silk reeling is commonly used in Chen style as a warmup before commencing Tai Chi form practice, but its body mechanics are also a requirement of Chen Style Tai Chi throughout the forms. In other styles, silk reeling is only introduced to advanced levels. Many schools, especially those not associated with the orthodox Tai Chi families, don't train it at all."
- Silk Reeling - Wikipedia