ok im about 80% sure what they did to get this effect.....if you take the volume knob on a guitar and turn it back and forth (on and off) while doing hammer-on's, u get this exact effect.
Eddie Van Halen also does this same thing in a van halen song called "Cathedral".
Try it out if your a guitarist, its loads of fun. (turn the volume knob off and then every time u hammer a note, turn the knob on, then back off quickly.)
sry im horrible at explaining it.