In order to understand how to achieve pinched (or artificial) harmonics, you need to understand how to achieve regular (or natural) harmonics.
Natural harmonics are achieved by touching your finger to a string (touching, not fretting) above a fret that marks a harmonic node point.
Next, pick the string and remove your finger from it so the harmonic can ring out.
Something you can also do is tap the harmonic node point with your fretting hand after playing an open string.
For example, you can play the B string open then tap the string with your fretting hand at the 12th fret and achieve a natural harmonic.
This is a lot like what happens when you play a pinched harmonic – just much quicker.
You see, there are harmonic node point all over the strings – there are even some beyond the fretboard near the pickups. these are the one’s you’re activating when you play pinched harmonics.
Pinched harmonics are achieved by picking a note in such a way that it slaps against the thumb of your picking hand after being picked.
This activates a harmonic node point around the pick ups, in a similar way to picking a string open and tapping it at the 12th fret.