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if you are trying to exclude the vocals, then you can sometimes do it by switching the polarity of one side of the stereo waveform. it works only if the vocals are dead center, by the signals cutting each other out completely. I've never tried it but you can read a lot about it on the web.

in my multi-tracking app, you can import a stereo wav file as two separate waveforms, one for left and one for right. you can then put them in two separate mono tracks, panning the left to the left, and the right to the right, obviously. then just flick the polarity switch on one of the tracks, and theoretically it should cancel any centered vocals out.

i've heard a couple of plugins try to do this, i assume they use the above method to do it. it obviously isn't done very well, because it mixes everything to a mono signal by the sounds of it. it works differently for different artists - for example, i tried using it with scarling, and it hardly worked at all. i also tried using it with led zeppelin (albeit the remastered stuff) and it worked quite well - there were still the presence of reverbs and delays in the mix though.

the weirdest effects occurred when used with nine inch nails. on the fragile's "we're in this together", it made it sound like a big piece of shit, being played through an AM radio with a bad reception. yet, on the downward spiral's "piggy", it does exactly what it says on the tin, a little too well - the only thing left in the mix is the drums! it's truly bizarre!
your milage may vary, and it depends very much on the way the source music was mixed down.