This a very tough task to accomplish.
Music is in stereo, meaning there are only two tracks or sides. It is not as easy as turning off the vocal track.
What is actually happening is that the software uses preset EQ settings to try to pull out frequencies that the human voice projects across.
This can also effect the sound of the music.
Another aspect is that with most recordings (used to be anyway), vocals are panned straight down the center, and vocal removing technologies used that to try to remove the center sounds, but some songs may have a stereo vocal and this will not work.
In short, some songs will work better than others when attempting vocal removal.
If I find the need to do this, I usually use the best preset from DJ Twist and Burn that works for me, then I use a very precise parametric equalizer to attempt further removal without killing the music too much.
The extra eq-ing occurs within Mixcraft.