“I arrived at this system by accident, and I don’t really know why it works. What seems to happen is that the third speaker reproduces any sound that is not common to both sides of the stereo – i.e., everything that is not located centrally in the stereo image – and I assume that this is because the common information is put out of phase with itself and cancels out.”

From the liner notes of Ambient 4: On Land.

(Source: http://music.hyperreal.org/artists/brian_eno/onland-txt.html)