How do Northwest Coast Indigenous mechanical masks reveal a second face?

Northwest Coast Indigenous peoples developed mechanical masks featuring movable parts that allow the mask to reveal a second face, which is generally a human image.1

These masks were created based on the belief that human spirits could take animal form and vice versa. Consequently, the makers of these masks fused human and bird or other animal features into a single object. By utilizing these articulating parts, the masks were able to act out entire legends during performances.1