As mentioned in the article on the initiate sangoma being fired from her workplace, here is the first of the explanations:
An ithwasa is a person whose state is best translated by the term ‘initiate’. The root of the word, -thwas-, has the verbal meaning of “emerge for the first time (as a season or new moon)” as well as “become possessed by a spirit (as occurs during divination)”.
In the applied form ‘ethwasa’, the verb also means “show signs of a changing state, as by spirit-possession to become a diviner or doctor”.
In another form of the root, intwasa is the hlonipha (need another explanatory blog here) term for inyongo or bile. Bile or inyongo in Zulu medicine has similar associations to choler in the Galenic system of humours, and is quite often spiritually significant.
So an ithwasa is, linguistically speaking, a person emerging from a chrysalis, on the path to becoming an adept channeller of the amadlozi (explanation to come). A person who is in this state is particularly vulnerable to ubuthakathi (to be explained) and possession by amandiki, and so must burn impepho (also to be explained) and chela (sprinkle protective medicine) in the areas in which he or she works and lives.
An important point here is that ukuthwasa, the process of initation into ubungoma (the practice and traditions and essence of isangoma), is a strictly controlled and ritualised process, and is at no point associated with the western negative conception of ‘witchcraft’ (or the isiZulu concept of ‘ubuthakathi’).