A friend of mine told me there was a way to import any song from an album into a track in any sound recording program, and then somehow reverse the polarity or make it "out of phase" or something like that...

which will, in many cases, almost eliminate the lead vocal.

Is this possible in Home Studio 6XL? If so, how can I achieve it?

Thanks

If you reverse the phase of a track and play it along with the normal track, that will effectively cancel the entire track, not just the vocals.
From the SHS 6 Help files:
 
Inverting the Phase of a Track

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A waveform's exact opposite is called an inversion. It is a shift of 180 degrees. A waveform and its inversion cancel each other out completely, so it is usually not desirable to have two track recordings of the same source if one is phase inverted. It can lead to reduced volume, lowered or distorted response in certain frequencies, or even silence in the case of two tracks which are exactly identical (i.e. cloned tracks).
Occasionally, for example when recording a source using two microphones, one of the microphones may be recording an inversion of the other, the resulting tracks may, to some degree, be cancelling each other out. SONAR allows you to invert the phase of a track to match another.
To Invert the Phase of a Track Open the Track view or Console view. In the track you want to invert the phase, click the phase inversion button .

OK

then how was this done, and how can I do it?

http://www.youtube.com/watch?v=lmO-9rR1-JM


and the author of the video says:

"http://www.youtube.com/watch?v=lmO-9rR1-JM&fmt=18 for HI QUALITY AUDIO.
I created this version by importing into Audacity and putting it OOP (Out of Phase). This technique usually removes most of the vocals, thereby creating a backing track only. Sometimes the bass is diminished by this process, so I have increased the bass levels after OOPsing it. Please enjoy!"