From David K. Jordan’s “A Medium’s First Trance:”
The initiation was held on a festival day, and many of its activities were combined or interposed with festival activities. Basically it included two vital parts. One was an exorcism of possible demons from Qīngshuǐ’s body —just in case. When the exorcism was over, Qīngshuǐ was still in trance. That was the final evidence that he was possessed by a god, capable of withstanding the exorcism, and not by a demon, who would have been banished by it.
The second important part of the initiation consisted of providing Qīngshuǐ (still in trance) with a sword and a ball of nails. Both of these are used by spirit mediums to mortify their flesh, causing blood to flow. Mortification of the flesh is common among spirit mediums in China, and it is considered to be a sign of divine presence that the mediums do not show any sign of feeling pain when they cut and puncture themselves.
A medium does not mortify his flesh until the community initiates him. When the community accepts him and holds the initiation, he is provided with these tools of his trade. After that, in theory, he can be called upon to go into trance when the gods are needed for advice or to participate in rituals. Guō Qīngshuǐ was destined no longer to be just an ordinary Taiwanese farmer. Once provided with the tools of his godly trade, he was also a spirit medium, and his life was changed forever.
I find the fact that initiation into the role of spirit medium can only be conferred by the community to be particularly interesting and significant. That doesn’t mean that spirit possession doesn’t happen to the uninitiated (it does), but in this tradition, to be considered a “spirit medium” requires recognition by the human community as well.