well, for what I know, it means that you tap with your right hand using one of your pickup as a vitual fret...
Vai does it, it sounds like a very high pitch. Ron thal uses a thimble to recreate a virtual fret, while hellecasters Will ray has a stealth slide on his right middle finger, allowing him to shorten the string while pulling off with his right hand, it sounds like a whammy pedal, but it's cheaper!!!