"Pinning" is where the dog's forefeet turn inwards towards the centre line when the dog is moving (also referred to as being "pigeon-toed") If you're interested in all these weird & wonderful canine terms, I would recommend you try & get hold of Harold Spira's Canine Terminology, an excellent book with really good illustrations.
Yeah they're great I always like the term "goose-stepping" movement - not v politically correct, but it makes me smile, imagining dogs marching around the ring I also like "frog-faced" !!