Vibrato. To get the right pitch of the squeel (pinch harmonic) right after you pluck, hit your thumb on it like he said^^^, but keep your thumb on it. Thats the sound you'll hear if you let it ring. A quick way to find the perfect sound. Some practice: No more tears by ozzy,played by the great Zach W. heres a quick tab for ya:
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|--------------------------------------------|
|--------------------------------------------|
|-0-3-0--6*-5-3-5-3-5--6*------------------|
|-0-3-0--6*-5-3-5-3-5--6*------------------|
|-0-3-0--6*-5-3-5-3-5--6*--5-3-5-3ph~---- |
.......... ...........
tuning is drop d or c(your choice), heavy distortion
... palm mute
* let ring breifly
ph pinch harmonic
~ vibrato
Another way to help do the vibrato, think of semi fast hammer ons and pull offs, then thats how fast you bend the string.