I happened to be holding my sketchbook when my young son was pulling a bell rope. Naturally he didn’t hold still for a second. But as I watched a couple of repetitions of the action, I realized that he kept returning to the same extreme or "keyframe" poses. So I switched back and forth between those poses, using the flash-glance method to hold them just long enough in memory.
As early as the 1930s, the Disney Studios hired instructor Don Graham to offer the animators a class in “action analysis” using models who moved through a series of poses.
Tomorrow: The Sultan's Elephant