If you do a search on pinched harmonics you will end up with quite a few similar questions on this forum
Anyway, if you understand the concept of natural harmonics, touched harmonics and pinched harmonics are only a small step ahead.
If you're fretting a string, the place where you can touch the strings to create harmonics shifts further along the string equally. So if you play any string at the 2nd fret, you can touch the vibrating string at the 14th fret to create the octave harmonic (on an open string this would be the 12th fret harmonic). Playing a fretted note, then lightly touching the vibrating string in a specific place like that is called a touched harmonic.
A pinched harmonic (also called artificial harmonic or making the guitar "sqeal") is basically doing the touch harmonic at the same time as picking the string. The most popular method (used extensively in rock/metal) is to let only a small tip of your pick extend from your fingers. As you pick the string this way, you let the edge of your thumb (which should now be quite close to the string) catch on the string at the place you wish to create the harmonic. It takes a bit of practice to get right, but it's quite a cool technique to use. Also, cranking up the gain/distortion on your amp helps a lot.
Remember that the places where to do pinched harmonics shift equally with the fret you're holding down. If you're fretting the 12th fret, the octave harmonic will be at the 24th fret. Even if you don't have a 24-fret guitar, you can estimate where it should be (somewhere around the pickups probably). Don't be affraid to try and pinch in a few different places to discover "sweet spots" for particular frets. Squeal away!
