Years ago, I made this instructional video called “Outside Tapping”. I might as well have called it “Hammer-ons From Nowhere” because that’s essentially what’s going on.
Hammer-ons from nowhere are notes you hammer on with your fretting hand on a string that hasn’t been manipulated before. Key here is to keep things sounding clean. You’ll find that a hammer on from nowhere is different than a hammer on in let’s say a tap sequence like “Eruption”, tap, pull off, hammer on, repeat. A hammer on from nowhere is actually the initial manipulation that makes the string sound, so it requires a different, firmer touch than a hammer on onto a string that’s already vibrating.
Check out Example 1, a riff that’s very close to the one I’m playing in the aforementioned “Outside Tapping” video. All notes on the G and high E string are hammer-ons from nowhere.
Example 2 is a little part of the riff I’m playing in the song “Echo (There You Were)” off the “The Psychomagic Protocol” album. More or less the same deal as example 1, only this time I incorporate open strings. Open strings allow you to speed things up without have to fret every note, but it also means you got to be even more careful not to ring out any unwanted notes or noises. Subtle damping of the strings with the “pinky side” of your picking hand might be the way to go here.