i think the thing you are talking about is called a pinch harmonic or an artificial harmonic. i personally not very good so i couldnt really explain it, but there are plenty of lessons out there about it
Quote by silly6-string
ya how do u do it?

First, pick an easy fret to do it on (I think the easiest is 3 on G). Then, choke way up on the pic with your thumb. When you pick, lightly tap the string with your thumb. If you do it right, it's pretty much just one fluent motion. Use lots of vibrado for optimal results.
You hold your pick so there is only a little bit of the pick sticking out between your fingers. When you pick the note, you do it like a digging motion, and let your thumb hit the string right after you pick it. They're very hard to learn at first, but are REALLY good for later on. I advise you take the time to learn them.