Arm and body "rotating"? -help

Hi, im wondering, this video, was created in toon boom studio, but how do they do the sort of “rotating under arm” if you now what i mean at 1:00, and the shortly after, he starts rotating, how is that done?
http://www.youtube.com/watch?v=9tI8RfN2iTA#at=114

link to the exact time: http://youtu.be/9tI8RfN2iTA?t=1m