It just sounds like a harmonic overtone coming through the original note. I love it when that happens. I believe Phil Collen's current model Jackson (PC-1) has a switch that does this upon demand (watch his demo on the Jackson website), but you can achieve it without any electronic trickery. Try adding a lot of mids to your eq. With your lead tone on, hold a note with a little vibrato until you hear it become a harmonic. It depends on so many things - intonation, pickups, eq, string action/height - sometimes it "just happens" by good fortune, and you can't make it happen again. Good luck!