Well, there's a certain truth to it... the only way this can be done is if you have a flawless digital copy of the tune and also a flawless digital copy copy of the instrumental version. And when I saw digital, I don't mean a wav rip of a vinyl, or an mp3 lol
If you have both these and invert the phase of them and then overlay the two tracks perfectly, then everything will vanish except the vocal.
You can make some half arsed attempts to do this by inverting the phase of one channel of a tune then mix it down to mono. You might be left with a vocal and a bit of background shit, but really it depends on how that track has been mixed in the studio.
The phase cancellation method is the only way this can be done properly, and I can't stress enough about how accurate the overlaying needs to be to make it work. They have to line up
exactly to the sample - and there's 44,100 samples every second. Even if it's a bit out it won't work which is why vinyl rips or mp3's won't do.
A good way of thinking about this is like I've said before in a couple of other posts about this subject... If you bake a cake, can you then take the eggs out whole?