Pinch Harmonics is almost the same thing as Natural and Artificial Harmonics. But to trigger the sqeal(harmonic) in pinch harmonics you play a tone with the pick and hold the left side of the thumb inline(behind) with the edge of pick.
So when you pick the tone the tumb should detonate the harmonic a milisec after the pick has left the string.
This ain´t easy to explain so if you don´t get it(You probably won´t cause there is not enought words to explain it, at least I don´t know them), have someone that knows the technique to show you.