To add to this, there is also a form of this known as as Astral Projection [1] and there is a great body of esoteric teaching about the common experiences of people who have achieved this. One may achieve access to a "place" known as the Astral Plane[2] that is said to be occupied by certain energies and entities, some of which can impart wisdom and some that can actually inflict psychic damage.

I'll leave it as an exercise for the reader to make up their own mind on the legitimacy and effectiveness of such practices. I think, like any practiced meditation, it is a matter of deep personal preference whether one sees through the mysticism or is stopped short by it.

[1] http://en.wikipedia.org/wiki/Astral_projection [2] http://en.wikipedia.org/wiki/Astral_plane