Put simply, an Underscore music mix is a version of the song where all the lead-instruments have been taken out. What's left is the basic backbone groove of the track.

Underscore versions are often very well suited for media presentations because the music provides an "atmosphere", but it doesn't stick it's head out and demand attention. This ensures that the actual message you're trying to get across isn't diluted or disturbed by any element in the music.

But Underscore versions are sometimes suitable for other things as well. Some people simply prefer the Underscore version. When you remove the lead instruments, you are left with a different perspective on the real back bone of the music. It can cast a whole new light on the composition.

Generally, unless otherwise stated, the Underscore version has the same length as the Full Length version.