+Akuma Kami I only bought this one- holdover favorite from Guitar Hero- but I love it! :)

I think they're harmonics, where you gently rest your finger on the string and then gently pluck it. The note still resonates even when you remove your finger from the string, it's really weird.

Make sure you lightly touch (don't press) the string right over the fret wire, as you strike the string remove your finger to let it echo out :) This is known as a 'natural harmonic'.