Tong-ki is a Chinese word (童乩 the Mandarin pronunciation tong2 ji1 represented in pinyin romanization is used here, in Cantonese, it is called San4 Daa2 神打) that refers to a person who experiences himself as having been chosen by a particular god to be his vehicle for expression here on earth.
Frequently a person who will become tong-ki experiences himself, at an early time in his life, to be coming under the compulsion of some god to be possessed by him. The person may well resist that compulsion. Should his resistance fail, he will from time to time enter into a trance in which he may beat himself with a nail-studded ball at the end of a cord and handle to the point that he draws blood from multiple wounds on his back. While in this trance state he will experience himself,and the members of his community of belief will experience him, as speaking the words of that god.
Usually the person performing this social and religious role will be called a "spirit medium" in Western languages. The phenomenon appears to have connections with shamanism and with religious Daoism (Taoism). However, a shaman is one who gains control of forces in the spirit world, whereas a tong-ki appears to be entirely under the control of forces in the spirit world.
External links and references
For a great deal of additional information, see the chapter on tong-ki