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From 2:32 onwards, can anyone explain to me the technique used to play this tremelo (over c major and minor chord in this example)?

http://www.youtube.com/watch?v=R6b77nNd1P8&feature=related

from what I can tell (and how I might play this) I believe the thumb is by its self and the other fingers are at the same time (so it's just a fast back and forth). to work up to this i'd try starting with two C's in octaves (thumb on C, pinky on C) and go back and forth using wrist motion. then add the other notes in between and keep your thumb by its self and hit the middle notes at the same time as the pinky.

hope that helps.