Mental projection is a supposed (or of course, experienced, to those who do it) sort of consciousness/spirit/intelligence projection from the emotional/astral to mental plane. Very skilled scientists, philosophers, theosophists, mystics, priest[esse]s, esoterists, magicians, psychics, or prophets say they are able to project to the emotional/astral world (or directly from it if the are not alive) and then (by completely calming their emotional processes and withdrawing their emotional senses) to the mental world. Like astral projection, mental projection is done during the sleep (before or after life, or in a dream dream,) meditation-contemplation, or 'psychic' (soulful) separation of the emotional/astral body also through the silver cord to the mental body. Of course, one could ascend projecting to the highest plane, such as with the gold cord, though note according to many esoteric philosophers, when projecting past the mental body one is no longer in a humanoid (human-formed) body but in a lotus or spherical/oboloid 'auric body' (or consciousness/spirit.)
Certain philosophers consider it better to try to also project from the emotional/astral to the mental world because it is more rational (though not necessarily over-rational as if it was separate; mental encloses emotional) and less prone to black magic and 'lower psychism,' but some philosophers even think it is better to project from the humanoid mental body to the causal and mental auras; reflections of higher formless consciousness in which causes, such as 'egotistic self-identification' (or anything a great prophet says is humanity's problem,) of the latter problems, are less prevalent. Basically it is best to strive to project into divine consciousness, not necessarily leaving one's lower consciousness, but becoming aware of spirit and divine will.