The consciousness transfer is a Na'vi ritual where a consciousness of a single being is transferred from one body to another.
The goal is to transfer the spirit or consciousness of the individual that it is being practiced upon into a "comatose" or empty body permanently by passing them through the, "Eye of Eywa." Only the Tipani and Omaticaya clans have been shown to perform it.
The Omaticaya perform the ritual by joining an entire clan's queues into one of the neural link locations on Pandora (e.g. the Tree of Souls) and performing a Terran equivalent of a large-scale ritual chant presided over by the Tsahìk. The Tipani Clan are also capable of performing the ritual, although Sänume appears to be able to do it alone in their clan's hometree.
It is, by all means, a risky endeavor in that if the subject is very weak, it can result in death.
It has only been known to happen successfully on two occasions. The first successful transfer was in 2148, when Sänume was able to transfer Able Ryder's consciousness after his link bed was crushed. Attempts to revive René Harper in this manner failed.
The Omaticaya Clan attempted to save Dr. Grace Augustine in 2154, but she did not survive due to previously sustained life-threatening wounds. Jake Sully went through the process the three days later, and became the second to have his consciousness moved permanently into his avatar body.