Two Forms is a racial ability available to worgen which allows them to freely transform between their human and worgen forms while outside of combat. The transformation is purely cosmetic, and entering combat while in human form will automatically return the player to worgen form.
The animation, in which the human's hands become clawed and their arms covered in fur, before fully transforming with a howl, required the creation of two new meshes, which the human model switches between before finally switching to the worgen model.