Watch the Bouncing the Arm online guitar lesson by Muriel Anderson from 50 Right-Hand Techniques You Must Know
Bouncing the arm is a secret I learned over time and I'm happy to share it with you! Just a little bit of bounce from the arm allows the fingers to do less work. The example here is an excerpt from "The Liberty Bell" by John Philip Sousa.
You can't really see the bounce from the angle of the camera, but you can hear how it helps to get a bouncier sound. You can also use a little bounce from the arm on slow pieces to allow the fingers to relax more.