Ay my man, I think you're talking about phase cancellation; it's what happens when you invert the phase on one side and (much like EQ adds to itself) it is supposed to subtract. However, vocal harmonics are pretty wide in the frequency spectrum, so even if that **** does work...it's probably not very good quality.

I read an article in scratch a couple of years ago when I was going to school and dood broke it down how to extract JUST the vocals from a song in protools...{separate the stereo song to mono, invert the phase on one, changing LPF's and mad filters and blah blah blah}...inspite of everything we tried in class, the end result wasn't anything to brag about.

Bottom line, it can be done, but you won't be impressed. Also I would recommend skipping the phase thing I mentioned, and just work your best effort with a good EQ... if you want a quality acapella, you gotta find it somewhere...if you can prove me otherwise, PLEASE do.

Peace