"Jin is a power in Chinese martial arts which is derived from muscles which have been energized by Qi to their maximum potential. In order to manifest Jin (i.e., power) effectively and efficiently, the Jin must first be stored. Storing Jin [in the Yi, Qi, and posture] is Yin, while manifesting Jin is Yang.
"Taiji emphasizes the Yin side and the Yang side equally, and can consequently balance Yin and Yang in the body and avoid unhealthy extremes. This is different from many other Qigong practices which emphasize the Yang side more than the Yin side. Practitioners who emphasize the Yang training will not get sick easily, but, because their bodies become Yang, they will age more quickly than is normal." *
* Dr. Yang, Jwing-Ming, The Essence of Taiji Qigong