Pinch Harmonics. :)
What you have to do is catch the string with your thumb as you pick. Just hold the pick so a little bit of your thumb hangs over the edge. Your pick and thumb will catch the string at almost the same time, creating a harmonic. You pick them where you normally strum, by the way, but you can move your hand around to get different sounds. Depending on where you hit them, they will have different pitches, and adding vibrato to a high pitched one can make your guitar scream like crazy, and thats only on a low string!