I haven't seen the video, but I assume it's the trick where you drop the bar, play a 4th fret harmonic on the G string and then yank the bar up to make it squeal?
Ages ago when I had a Dean ML (was a BIG Dimebag fan years ago) I read a magazine interview with him where he was talking about how he did all these weird effects.
I learned them all :lol:
1 of my old favourites was to play the 5th fret harmonics on the G, B and E at the same time, dive these with the bar and the do the G string 4th fret harmonic squeal on the way back up :D