The reason is that from Twin Diamonds, the ends (points) are considered to be Ends of Lines, and when a shape-changing call is directed to the centers, "square breathing" causes the Ends to laterally slide toward or away from the center.
From Twin Diamonds, if the Centers do a Lockit, the Ends slide away from the center, but remain close to each other (in Tandem) as if they were Ends of Lines. This results in the "dogbone" formation.
