For example here at ~:41 secs. http://www.youtube.com/watch?v=B-65K8TI-3s&feature=youtube_gdata_player
With various amounts of pressure you can achieve different sounds
It's like refreshing the page or pressing the reset button on your left hand. I don't know, it didn't effect the sound, it just looks different.
It can, though I couldn't watch that video try it on your guitar with a high gain setting on your amp it can be real fun
Ok. So not really a special technique really. Just a "thing" some guitarist do. Ok. Thanks guys!
I've always wondered about that myself.