Originally Posted by: renyou also have to change your grip on the pick / angle of attack.

Going for a pinch harmonic, you want to get the side of your thumb to touch the string just after you pick it.

The guitar string may vibrate primarily at one frequency, but there are many little different frequencies ringing out at the same time (For example, if A is 440hz, there are frequencies of 880hz and 1760hz ringing out of the string even though you're playing the 440hz note, and this is true with all sound as well). By touching the string at certain places as you pick you're muting out the 440hz but allowing the 880hz or 1760hz to ring out louder. (Playing any open string with your fret hand finger lightly touching the string over the 5th fret is a profound example of this).

This is also the concept on how MP3s are able to srhink the size of an audio file (by removing all the frequencies you can't hear outright).

To get the sound, what you need to do is fret the note you want to squeal FIRST, then you have to adjust yoru picking hand position (as in where on the strings you pick), then pick the note and lightly tap your thumb to that special spot as you hit the vibrato. This takes LOTS AND LOTS of practice so don't get discouraged in the first five minutes. The end result is FAR worth the effort it takes to perfect it.