"When you pick the string just right, a higher pitch other than the fretted note is sounded. This higher pitch is an overtone, or harmonic, that stems from the overtone series related to that note. Indicated by the abbreviation P.H., the pinch harmonic is a fantastic expressive device to use when playing a solo or melody. Much beloved by rock, blues, country and metal guitarists alike, the pinch harmonic has been used to great effect by such legendary axemen as Roy Buchanan, Billy Gibbons, Eddie Van Halen and Zakk Wylde."
Playing a string harmonic is a string instrument technique that uses the nodes of natural harmonics of a musical string to isolate overtones. Playing string harmonics produces high pitched tones, often compared in timbre to a whistle or flute. Overtones can be isolated "by lightly touching the string with the finger instead of pressing it down" against the fingerboard. For some instruments this is
"The way I and most guitarists produce a pinch harmonic is to grasp the pick close to its pointed tip with your thumb and index finger. You then pick a downstroke, intentionally allowing a bit of the fleshy part of the thumb to graze the string at the same time."
"In short, pick squealing, or pinch harmonics is part of the reason why a lot of people started using copious amounts of hairspray and dressing in very tight clothing during the 80s. However, pick squeals, or, in less cool words, pinch harmonics, can be used in a much broader spectrum of ways than just heavy metal, and the technique was originally probably first used by blues players of old."
"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."