Resh Lakish says that the dangling limb is similar to a limb of a fetus protruding from its mother's womb, and it is subject to the disagreement between Rabbi Meir and the Sages. Rabbi Yochanan says that in the case of a dangling limb there is no disagreement, but rather it is considered attached to the body of the animal, and shechitah makes it permitted for consumption and free from impurity - except that the Sages later forbade to eat it. By contrast, if the animal with a dangling limb dies by itself, the limb is considered to have been detached before death, and its impurity is less than of a nevelah. How so? A piece of meat separated from a torn limb is pure, while a piece of meat separated from an animal that died by itself is impure.
Art: William Huggins - Rams and Ewes in a Landscape