howNatural: touch (don't press) directly over the 12th, 7th, or 5th fret wire and pluck — you're isolating the string's own overtones. Pinch: choke up so the thumb's flesh grazes the string a split second after the pick; move the picking spot to find different overtones.
musically, it's forChimes, bells, and the squeal. This is the Overtone Argument made audible — the physics essay with the amp on.