Below is an excerpt from a post by Jim Roach at the Classical Tai Chi Blog. The full post may be read here.
Zhan Zhuang for Taijiquan Practice. The Wuji Positions allows the practitioner to relax the mind, while, adjusting, aligning, and balancing the body to produce correct postures. Zhan Zhuang Training, on the other hand, strengthens the tendons and ligaments, aids in balancing, teaches the muscles to relax, and identifies weaknesses not noticed while practicing the Form. This is important in helping to identify the proper placement of the heel and weighting of the empty foot.
The Square Form of Classical Taijiquan, brush knee movement is being used to illustrate how Wuji and Zhan Zhuang can be applied at those stopping points for each position. Applying these training methods in addition to form practice will help the student in developing strength and proper form. Wuji is defined as nothingness, the beginning before intention and movement. Wuji is discussed in many Taijiquan Books written by both practitioners and masters alike. These writers mainly address Wuji in the Preparation Posture and/or the Closing Posture of the Taiji Form. As such, most readers are left to believe Wuji is only accomplished at the beginning and ending of the Taiji Form. However, this is not so. Wuji is practiced during every posture, that is, every posture begins with Wuji, moves into Taiji, and returns to Wuji.
Zhan Zhuang (standing like a stake, standing like a tree) Training is a way to relax both the nervous and muscular systems simultaneously. This is accomplished by combining exertion and relaxation simultaneously. Breathing is done by inhaling and exhaling gently through the nose while keeping the mouth closed and relaxed. The chest, stomach, and hips are in a relaxed state. Zhan Zhuang helps with the identification of the energy flow in the different positions and trains to keep the localized nerve activity dormant (Forum 6); as well as, strengthening the yin side of the posture for strong rooting and building power (Forum 7). There is no set time limit in Zhan Zhuang Training; however, the seasoned practitioner has been known to hold the positions in excess of twenty minutes. Some have claimed to be able to hold the positions for hours. It is important to remember, that as the tension builds in different parts of the body, to tell yourself to relax. (RELAX, RELAX, RELAX) Start with short time frames and increase the holding time slowly.