Question: Where specifically on the arm/hand should rapid picking speed come from?

Answer: 
There is a famous part of Eruption that quotes Etude No 2 by Kreutzer ([reference here](http://en.wikipedia.org/wiki/Eruption_%28instrumental%29)). For that style of picking, it is important to **lock** the wrist, and use the forearm in a tight, fast pattern (yet staying relaxed ;-)

[Here is a link](http://www.youtube.com/watch?v=K4lrAxUuwkk&feature=related#t=3m12s) to the quote, which shows Eddie's technique
A fast picking speed comes from mainly the hand both working to move the pick quickly. However the bicep does a little work, keeping the hand and arm steady to provide accurate picking.