I just suggested you to write this topic to that other forum, where the discussions isn't ONLY about Bodom, then it would have been very ok.
And to answer your question...
Here's what you do:
Pick a note (for example 9th fret on the G-string) and then right after you've picked the note, then touch very fast with your thumb to that string very softly and then you'll get that sound.
I think this is the thing you wanted to know, I guess.
-Antti