I'm looking at songs like Daron Malakian on the SOAD song Chup Suey or that Head dude that got dunked in the Jordan river :) on the Korn song Got the Life where they play one of those, I guess you'd call it a fifth cord with the two octave notes and the fifth muted.

I kept slowing the pace down on the Sytem of a Down video to see how he is doing it, and it looks like it's all picked from the wrist with a little bit of rotation, but mostly flat, moving back and forth.

I can sort of play quick like that if I bend my wrist almost at a right angle, lifting my forearm way off the bridge, but that's ridiculous and has no control whatsoever.

Is it mostly just a matter of practice? Or is there a trick like barely hitting the strings? I've seen in some other videos where people strum fast from the forearm, but I don't think that's what I'm looking for. Any help would be appreciated.