Phrasing

Phrasing or phasing? The disc jockey article links here, and talks about phrasing, but this explanation is about phasing.

Phasing, in this context, is the playing of two copies of the same piece of music, almost exactly in sync, and mixing the two signals together. By slightly changing the pitch of one of the copies, a phasing effect is heard which sounds like an overhead airplane. For this reason, performing phasing is often referred to as "doing an airplane".