This is just outlining a little trick I’ve found for placing instruments outside of the stereo image. This is useful for ambience sounds that don’t require much active attention but help to fill out the sonic landscape of the track thats being mixed or if for any reason you want to throw a certain image really wide in the mix.
To achieve this effect we double the track that has been recorded with an identical recording pan them in opposition to the one another and then flip the phase of one of the signals. The reactant effect is sound that appears much wider than before. This happens because the signal that has been flipped causes phase cancellation in one half of the stereo field forcing us to perceive the sound as very wide in relation to the entire stereo image.
Jordan Vaughan: Creative Sound Design 