Not so much a beginners technique, it is what flashy pants lead guitarists like Slash and Nuno use for super fast solos, it requires a very stable right arm and wrist. Because there is no plectrum or plucking going on you don't hear any form of attack on the strings, just instant note changes. Give it a go with medium overdrive and a touch of flange or chorus, it can sound almost like a square-wave synthesizer.
A little suggestion for anyone who uses it already: Okay, so it helps you make double speed note-by-note solos, but have you ever tried making a powerchord or similar, and fret tapping along its multiple strings in various places like an arpeggio? It sounds great for main song riffs rather than solos and when you play it on an ambient clean sound it can sound pretty weird and trippy due to no plectrum attack- just note swapping.
Here's some pretty damn advanced examples by the amazing Micheal Berk on YouTube.