It only really works on older songs were the vocal (along with the kick, snare and bass) are in the middle and are in mono. Heres a vid on youtube showing you how to do it in Audacity.
Sorry read your post properly and and realised you were on about Phase reversal/inversion. This works best if you have an instrumental track and the full track.
What you do is line up both track precisely and invert the phase the instrumental track thus cancelling all instrumentation leaving only the vocals.
Its far from perfect thoug as it leaves other artifacts like certain hi hats frequencies and other unwanted noise.