what 'MikeP' explained is a another very cool Technique that is over used by Dimebag, you can hear it at the end of Cemetery Gates ,
but actualy Dimebag doesn't strike the string with the pick at all, he just do this:
1 - dive the bar
2 - hit (like hammering but without fretting) the harmonic with
your left hand middle finger and mute other strings with the
rest of your fingers
3 - raise the bar to the pitch you want
it's not an easy to learn technique, specialy the muting thing, but it sounds cool and worth the effort