Harmonics or Feedback Techniques

Can anybody give me som hints as to how to perform those squealing vibrato effects like Ozzy's No More Tears? I've seen Zakk Wylde do it all the time and i thought it was from pinching a string or using a reverse whammy tecnique by tightening the string rather than loosen it. It doesn't appear to be either. Any ideas?

Throw your guitar down a flight of stairs with the amp on. That'll do the trick almost everytime. :D

........Nope, that didn't work. Anyone else out there who knows?

bridge pickup with lots of overdrive/dist.

it really helps to have humbuckers too

you also have to somewhat search for the sweet spot which will give you the loudest clearest pinch harmonic