My favorite picture of Sonny is of him standing in front of a wall of weapons, blades mostly, but some sticks too. Apparently he thought he looked like a shopkeeper in the picture and did not really like it. Ah well.
Every blade on the wall was something he had made or modified, and he had a wide assortment of blade shapes for us students to play with.
Sonny thought that every weapon had a personality because of it's design. More specifically that the balance, the curvature, the re-curve, the width, the size of the tip, the flare along the length, and the handle design, all played a part in the way it moved.
Most classes would start off with one blade shape, but over the course of the next hour, he'd switch up blades maybe 3 or 4 times, returning to some later in the lesson.
Why did he do this?
He thought particular blade shapes would give the student particular insights when used. For instance, curved blades slice better/easier, than straight ones, just because of the way human arms are attached to the body. Narrow blades with long tips move faster in space and feel more nimble, 'flicky' even. Long blades 'turn corners' better than short ones, even more so with a curved back tip. Handles that turn down at the end move the tip differently than straight handles.
Every blade changed the way you cut, the way you blocked, and so how you moved with it. Basically, the whole geometry of the interaction, where was safe, unsafe, in range, out of range, would shift dependent on the blade design.
But instead of inventing different systems for every blade, he used them interchangeably to teach concepts, with the idea that eventually you would be able to use any blade to do anything. You'd begin to feel the curved slice in the straight edge, the fast tip in the wide blade, the fulcrum in the straight handle.
He wanted you to understand that it was the body movement that changed, not just what was in it. So not only would changing the blade make you move differently, but that by moving differently, you could effectively change the shape of the blade in your hands ....
And just because I'm throwing some old posts back up, here's one about context and blade design.