This technique is used quite heavily in hard rock and metal with players like Zakk Wylde leading the way. However, it has also been used by blues based players like Billy Gibbons of ZZ Top to get a more nasty and biting sound out of the strings.
The technique basically requires hitting the string with the pick and flesh of your picking hand thumb at the same time while over a harmonic node point on the string.
The video is pretty self explanatory and after you know what to do to create this sound you can more easily develop a good enough feel for it that you won't think about it very much at all. It will just become a natural part of your playing style.
Make sure that you watch the pick angle and everything in the video, it makes all the difference in the world.
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