The tai chi ball is used for body conditioning. It strengthens the arms, torso, spine and lower back. The wood ball weighs between 2 to 10 lbs depending on what size ball you use. The motion has a cumulative effect in gradually building your physical body strength, without tensing too much, so your qi circulation can still be abundant.
For strength training the variables used are: 1) the weight of the ball, 2) the number of repetitions of each movement performed, 3) the number of movements performed in a training session, 4) the depth of the stances (lower = harder), 5) the speed of the movements, and 6) the focus, concentration, and mental intentions used in each movement.
In the beginning posture of the tai chi ball qigong practice, the mind should be calm and empty, and the weight evenly distributed on both feet. This state is wuji. Your mind starts to lead the body into the movement, internal (yin) and external (yang).