the term you used, "pressure harmonics", is used loosely in this technique. It's just a name to specify exacly which kind of harmonic the guitarist is going for. But, the technique can be wrapped up this way...

For example:
1. Hold your pick between you thumb and middle fingers.
2. (for simplicity's sake) you will actually pick the string above the 15th fret on the high E string.
3. With your right hand forefinger touch the string directly above the 12th fret. Above the FRET not the space in between frets.
4. Lift your forefinger immediately after sounding the picked note.

I'm sure I made it sound a lot harder than what it is. To be honest, this technique borders on "useless" considering pinch harmonis are exactly the same thing only using the meat of you pick hand thumb instead of the forefinger. But with this technique you can look flashy to your friends. Keep in mind: I used the 12th fret as an example. These harmonics are all over the neck. For that matter, they are above the pickups too. Try it out, and if you're still having trouble, let me know!