The 纏絲工()[silk reeling work]demonstrated here is based on the same principle of movement in Classical Tai Chi we described in the recent "Hand follows foot..." Blog. The principle being one which folds the body at the spine. In this instruction however, the feet/legs are not moving. Unlike "Hand follows foot..." however, one side of the body is truly kept as still as possible while the other side is moved. An often used simile is that of "folding the body like a book". Master Stephen Hwa here uses the phrase "folding like a door"
The exercise demonstrated here is a "pure" silk reeling, since the movement can and should be continuous with no interruptions or breaks.
In particular you should look for a sensation of "folding" at the lower back. Gradually, one notices this sensation moving from more near the upper back to the lower as they become more proficient with the exercises.
As you see Master Hwa do here, you can use the free hand to "feel" exactly where the sensations of "folding" are taking place.