That quotation and the reference to Wikipedia do not explain the way an autograph is brought onto the film.
It seems to have worked this way:
-) the backing paper is of a kind that has reduced opacity
-) between film and backing paper is a carbon paper with the paper itself being transparent, with the carbon side facing the backing paper
-) the space for applying the autograph must give way for the backing paper to lift off from the carbon paper
-) whilst inscribing with a stylus the waxed carbon is transferred to the backing paper
-) after inscribing there is enough play between carbon and backing paper for light to enter via the two papers onto the film