On the guitar, pinch harmonics is the act of lightly touching a string with the edge of your thumb/palm (picking hand) after
plucking it. Similar to natural harmonics except you're not playing off a
fret. If you use a lot of
distortion and gain it produces a squealing effect.
A lot of guitarists use pinch harmonics, most notably
Zakk Wylde, but also players like ZZ Top,
Dimebag Darrell, and even
Steve Vai.