It's called a pinch harmonic... and if you want to make the note sing like Dimebag does just combine the use of a floyd rose... after you execute the pinch harmonic rock the floyd rose back... the farther back the higher the note. A lot of metal guys use this like Zakk Wylde, DIME, Hammett... so on... but Satriani and Vai use it ALL the time. If you're learning "Cemetary Gates" you definitely need to know how to do this because everytime you hear that screaching higher pitched note... it's a pinch harmonic. Practice Practice Practice and you'll be doing it all the time I promise you! haha. A tip on learning how to do this... hold your pick so that just a little bit of the tip is sticking out. When you strike the note don't press the skin of your thumb onto the string... you just want to graze the string giving it that harmonic sound. I've never heard of anybody holding there thumb on the string to do pinch harmonics... but whatever works for you. Also, a good pick to use for playing stuff like that are the Jim Dunlop Stubbies.