Invert reflects the selection about the time (horizontal) axis. The selection is essentially turned upside-down. This produces no noticeable effect in mono sounds and has a slight effect in stereo sounds. Inverting a single channel of a stereo sound produces an "in" or "out" effect.
Inverting can be used before mixing so that the two sounds are subtracted instead of added.
See Also: Channel Mixer, Dynamics, Channel Menu