shamanistic strength

A shaman doesn't regress because he or she is intrinsically "sick".  It is not that simple.  Strength of mind is also a vital part of shamanism.  So, shamans are more likely to be those of intrinsically strong minds if they do turn out to be shamans after all (ie. if they recover).

According to Anton Ehrenzweig, a lack of access to consciousness where the ego is de-differentiated signifies schizophrenia. One must be able to dissolve one's stress by temporarily de-differentiating the ego from the field of one's being, so as to regroup with creative resources, rather than pathologically splitting and projecting.

Social or political oppression, or unexpected violence causes the future shaman's temporary regression. They do not stay in a regressed state but learn about the depths to the psyche from their experiences.

It can also be an accident that causes the wounding (as per Frieda Kahlo), but generally shamanism is associated with the lower classes of society who may be subject to systematic oppression (see Michael Taussig and the situation of the Columbian indians). It must be an inbuilt mechanism of the human mind that when put under a state of extreme stress it aims to return to the safety of the womb. (The post-Kleinians make much of this, although not in terms of shamanism).

Forces of oppression can be seen to be responsible for keeping some members of society in a state of immaturity in relation to the dominant classes (and gender). This would give them automatic closer access to the magical pre-oedipal level of consciousness -- a realm of transformation and mystical consciousness. But the intrinsic strength of the minds of some of the oppressed classes would result in the strange occurrence that these individuals do not descend into madness never to return, but do return after this baptism, with all sorts of things like an insightful social critique, an enhanced imagination, and enhanced survival skills. That is because their "madness" was never intrinsic to them, but was caused by direct pressures from the outside.

Such pressures turn the otherwise healthy and vibrant mind inwards for a while, so as to get to know itself in all of its imaginative complexity, thus releasing blockages and repressions that would otherwise cause neurosis.

That is why the shaman's "wounding" is often so beneficial to himself as well as to his community. It provides them with insights as to the meaning and nature of the ills within any particular community -- and what can be done to remedy these ills.