Reply to Skrillex- Devil's Den : Help making synth sound

I was wondering if anybody could help me. I am trying to figure out how skrillex does this back and forth synth sound right before the first drop of Devil's Den.

http://www.youtube.com/watch?feature...HLc4FWwA#t=56s

If this link doesn't go straight to the sound...It starts at 00:56.