When the user is within physical contact with the mask they are able to use any power that has been used by any being, mask or machine at anytime, anywhere, but someone or something has to have used the power for the user(s) to be able to copy it. Ex. Mind control, reanimaton, Iden, elemental powers, etc.
When in the time before Mata Nui it was in the shape of an Arthron, which then changed to the shape of a Phantoka Kakama Nuva when Mata Nui came to exist, then changed into the shape of the head of Pridak when Oron Nui dissapeared. The shape changed again into the shape of the Great Mask of Mimicry when it came in contact with Hordika Venom, around the time the air breathing Barraki and the Toa Inika came to Arkon Nui to hunt the mask.