Has anyone mentioned "diffusion" yet? (Doremus used "infuse"...)

The reason for agitation has little to do with chemical "reactions" but a lot to do with helping the chemicals diffuse into the gelatin matrix of the film. The agitation ensures that fresh developer solution is brought into physical contact with the surface of the gelatin so that fresh developer gets into the gelatin and it helps remove development byproducts from the gelatin faster than if there was no agitation.