One of the most interesting (and most challenging) techniques of playing a guitar is effectively utilizing a pinch harmonic (the aptly named "squeal"). It is typically used in metal music, as the heavy distortion used in amplifiers can greatly increase the sound of the otherwise subtle harmonic. Eddie Van Halen, for example, used this technique often, and well; an overwhelming number of his solos feature it. Take, for example, his iconic solo in Michael Jackson's "Beat It" (this obviously is a cover, but is nearly identical to the original):
At around 0:24 is that awesome, squealy pinch harmonic that metal guitarists and enthusiasts love. I think it's pretty awesome myself. So what's "physics" about it? More than you'd think, and more than I'd thought. Obviously, guitar strings have a fundamental frequency when they are being played open (no fingers on any frets). They also have overtones when frets are being played (the harmonics higher than the fundamental). The strings even have those fun little nodes, points in the standing wave of the string that have the minimum amplitude. All of these are vital to the pinch harmonic.
The technique involves lightly touching the thumb to the vibrating string, immediately after the string is picked. By doing this, and essentially "interfering" with the string's vibration, all fundamental frequencies and overtones are muted - unless the frequencies happen to have a node on that particular fret. Something interesting to note is that to make a pinch harmonic exactly an octave higher than the fret you are playing, you must pluck the string directly between that fret, and the bridge of the guitar. Doing it somewhere else will yield a different, yet equally cool, squealing sound.
Guitarists like Eddie Van Halen and Joe Satriani use the "divebomb" - using a whammy bar to alter the bridge during a pinch harmonic, thus drastically changing the frequency of the harmonic. This can only be done with very heavy distortion. For a textbook example, I'll use Joe Satriani's "Satch Boogie" - check out 0:32 to see what I mean.
Pretty cool, huh? So next time you listen to a Van Halen solo, or your favorite metal shredder, or even good ol' ZZ Top, keep this in mind - I'm sure you'll notice this technique!