Body Structure

For both qigong and taiji the lower half of the body needs to be stable and grounded while the upper half of the body should be open and fluid. If the base is solid then its possible to use structure rather than tension to hold the body.

The only trick is to remember that stable is not the same as immobile, and fluid is not the same as limp.

Here are two statements by Wang, Zong-yue (trans. by Yang, Jwing-ming):
"Stand like a balanced scale, (move) lively like a cartwheel"
from Taijiquan Classic

"Calm like a mountain, move like a river"
from 13 Postures