The Tai Chi Notebook. An excerpt is below. The full post may be read here.
The real lesson of Cloud hands though is to turn the waist. It’s a
common mistake to not turn the waist enough in Tai Chi, and I find that,
for the beginning student, a breakthrough in this area often only comes
about through a study of Cloud Hands as an isolated technique, repeated
over and over. Notice that if you removed the stepping from Cloud Hands
then it wouldn’t be a million miles away from the stationary
silk-reeling exercises that go along with Tai Chi, with one hand
performing a clockwise circle and the other an anticlockwise circle.
Indeed, performing Cloud Hands repeatedly can serve a similar function
to basic Silk Reeling exercises.
As it says in the Tai Chi classics, the body should be directed by
the waist at all times, so as you turn from side to side in Cloud Hands
(let’s not worry about the stepping for now) your arms should only be
moving because your waist is moving. If your waist stops, then your arms
should stop too. This especially applies at the crossover points, when
you’ve turned all the way to the side and the arms swap position, so
that the lower one becomes the upper one, and vice versa. This is the
point that beginners usually drop the principle and use some isolated
arm movement, instead of keeping it all coming from the waist. It’s
usually a revelation to the student here that you need to turn the waist
a little more than you think you do to keep it as the commander of the
movement – you should feel this using muscles in your lower back you
normally don’t reach.