The original name of taiji ball or Tai Chi ball, is “yin-yang taiji ball qigong” (yin-yang taiji qiu qigong). The training theory originates in both both Chinese martial arts and laymen societies, and each style of practice was kept secret, and only passed down to selected and trusted students. Some may have had a knot or indentation filled with a plug of the same wood. This is skilfully done and considered normal, in smoothing out for a full, spherical finish.
The general training purposes of taiji ball qigong training in martial society, was to create greater quantities of qi in the body, and ensure it would flow when needed in battle. The training methods not only support the circulation of qi, but also strengthen the torso, especially the spine and lower back, and condition the bones, muscles, tendons, and ligaments required for combat.
For the internal methods of practice, such as Taijiquan, Baguazhang, Fo Zhang (Buddha hands), and hu die zhang (butterfly palm) – balls made from wood are preferred. This is because, with wooden balls, the qi between the palms can be felt and directed more easily than with balls made of metal or stone. Here are some beautiful taiji forms from the ZhiNeng QiGong system: Set 1 with single ball & Set 2 with double balls.
Modern use has further evolved, with creative uses that challenge your strength and proprioception in a constantly changing environment. Fighting Monkey are pioneers in such methods using Dragon’s Pearl heavy wooden balls to push you further. Also, see primal.6 and his own unique interpretation of tai chi ball techniques.