The Yinshung are not necessarily a separate race due to a difference in biology, but rather are classified as such due to their distinct nature and common origins. When a mortal with some degree of chi-mastery (or has great potential) is pushed to their breaking point and survives through a massive amount of trauma, there is a chance that they undergo a transformation into a Yinshung.
This transformation does not necessarily change the outward appearance of the individual, although the sudden influx of power makes such changes possible. What defines a Yinshung is the sudden single-minded focus on a certain goal or ideal, accompanied by a massive spike in their physical strength and mastery over chi.
It is also important to note that Fuku, has cursed the Yinshung as a whole for their tendency towards conflict and capacity for violence. This has forced the Yinshung to largely go into hiding, with elder Yinshung identifying the newly transformed and whisking them away to secluded hideouts. At these hideouts Yinshung test their newfound strength and recover from whatever trauma has affected their lives, before finally heading back into the outside world. However, due to the single-mindedness of many Yinshung, infighting and conflict can arise in these hidden locations, leading to frequent changes in location or self-imposed exile in order to prevent further conflict.