Autographic film was introduced in 1914 by Eastman. It consisted of a spool of ordinary film with a backing paper which was not quite light-proof. Between the backing paper and the film was a strip of carbon-like paper. The two together forming a light-proof barrier to the film. A door in the back of the camera could be opened which exposed the backing paper, by writing on the backing paper with a stylus the carbon layer was disturbed and made no longer light-proof. The door was held open for a few seconds to expose the writing which then appeared in the negative.
(via Kodak roll-film & film-packs - Antique and Vintage Cameras&Craft! Bang! Boom!)