Flips the phase on the audio. Vocals should pretty much sound the same. You are probably hearing a psychological effect. Very useful if you have two out-of-phase signals, and you want to play them at the same time, without them interfering with each other. The most common and simple example is a snare drum, miced on the top head and the bottom head. The top head of a snare drum is by nature 180 degrees out of phase with the bottom head... so if you don't flip the phase on one of the mics, you end up with weird cancellations, and a crappy snare sound.
