Traditional Animation: How to Flip and Roll animation.
When I was at Cal Arts, I thought this was one of the most ridiculous things for the instructor to teach us until I tried to do it, and man was it difficult for me at first. I think of that to this day every time I get a crop of new students in my class about to learn the principles of 2d animation. Below are a couple of great videos that were made by some animation vets. The first video came from Animator Dan Keunster’s Animation Boot Camp Site. He made this great video which covers how to Flip while inbetweening in Traditional Animation.