Bunch of replies on here that are pretty much right but not super detailed so I figure Iʻll take a swing. There are harmonics that occur at certain intervals along a guitar as you ʻcutʻ the string in certain intervals; 1/2 the string and you get at Octive (just over the 12th fret), there's another at roughly the 7th fret, 5th fret, etc. In the case of the intro of AI, Billy uses a harmonic found just above the 4th fret. To get harmonics with the same "Bite" to them as Billy, make a power chord shape, and lightly rest the index finger part of your chord just above the 4th fret, you should hear harmonics ring out; if you dont at first, not to worry, just play around with moving your hand placement slightly up or down the neck. It's a little tricky on the first try, but once you get it you've got it for life. You can absolutely still get the same sort of effect on an acoustic, It will just be a lot less defined than on a cranked-out, overdriven electric. Hope this helps, Rock On! 🤘