Communication with the spirit world through a medium, an individual believed to possess the special ability to communicate with spirits existing in another dimension, is referred to as spirit channeling. Overshadowing is the term used to describe a situation where the spirit dominates the communication. "When spirit links with a medium, the spirit communicator exerts various degrees of control, or overshadows the consciousness of the medium to a greater or lesser degree," says the Rev. Simeon Stefanidakis of the First Spiritual Temple in Brookline, Massachusetts.
Overshadowing occurs to some degree in all channeling. The medium may retain some level of consciousness within the communication or may be completely overtaken (overshadowed), with the spirit controlling all speech and action, devoid of any translation by the medium. This does not denote any form of possession, as mediums typically control the level of overshadowing which they will allow. In some instances, the voice and mannerisms of the medium will change and reflect those of the spirit instead. This trance state is considered the deepest form of overshadowing.
Source: Mark Bingaman