The legendary guitar player, Roy Buchanan introduced the guitar playing public to the fascinating technique of pinch harmonics. This is a technique with a number of uses but is mainly used by metal guitarists to dress up the sound of a solo with a squealing sound. Steve Vai’s “Bad Horsie” is a well-known example of the use of pinch harmonics.
The way to prepare to play pinch harmonics is to hold your pick so that the pointy bit barely peeps out from between your thumb and your first finger. This sets your thumb up to brush against the plucked string right after the note is sounded. To produce the sound you are after you have to pick the note at a certain place along the length of the string. The exact spots that give good pinch harmonics are different for every guitar, so try the technique along the length of the strings, starting right near your pick-up, till you are comfortable with how you are holding your pick and the sound you are getting.
You need maximum distortion and maximum volume for effective pinch harmonics, and as with guitars, each amp will give you a different result.
Here is the ever-helpful Justin Sandercoe with a video guitar lesson showing you exactly how to play pich harmonics:
