Because they are unable to reach the Spirit Realms, the Hsien are forced to collect and gather Chi in order to survive. Yugen is undifferentiated (not Yin or Yang) Chi produced by the interaction of a hsien’s Chi with the Chi from the Yin and Yang worlds. This is only gathered at Dragon Nests and during religious festivals -- basically, places and times when the Wall is weak.
Yugen is similar to the Glamour their western cousins use, but it can't be inspired through creativity or similar feats.
- CtD. Land of Eight Million Dreams, pp. 97-98.