What is a pinch harmonic?
Definition
A pinch harmonic (a "squealie") is the high, screaming note you get when the edge of your thumb brushes the string right after the pick hits it. When that happens at the right spot on the string, the normal note is cancelled and a high overtone jumps out.
Why position matters
The squeal only happens at certain points along the string, called harmonic nodes. Where you pick decides which overtone you get, so moving your picking hand toward the neck or bridge changes the pitch of the squeal. Gain and a bridge pickup make it louder, but the placement is what makes it happen.
The full lesson, step by step: how to do pinch harmonics.