Marymac has sent me this link to a fascinating collection of photos of dogs in mid-shake. The heading of the article about the soon-to-be published book of photos is 'The Portland Shake'. I love that name.
I love watching Penny shake, but I don't have the patience or the camera to catch the movement in such detail. I've tried often, but usually I get just before the shake, or just after:
I think this is my best one so far: