"There has always been a good deal of mystery surrounding the pinch harmonic, or, as hip players like to call it, “pick squeal.” A pick squeal is simply an artificial harmonic, or high-pitched sound, produced by choking up on the pick and allowing the thumb or thumbnail to catch the string in just as it is picked. The result, of course, resembles a squeal. Or a squawk. Or a scream. (It could take several tries before you get the desired "s" word.) Anyhow, what was once the domain of blues-rock string benders is now a staple for most metal guitarists."