Comment 2 for bug 324032

I believe it's just an effect where you remove whatever signal is common for both right and left channel of a stereo track. Vocals are often panned center in the mix so they will be removed using this technique. It's far from perfect though.

Invert the effect and you end up with acapella.

See: http://manual.audacityteam.org/man/Tutorial_-_Vocal_Removal_and_Isolation