Feedback effects

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?

Zakk uses his wah pedal along with pinch harmonics to get that sound.