Double-exposure: Demy filmed two movements on the same film: a first time, the sleeping Prince; then, after having rewound the film, the Prince getting up to go join Donkey Skin. Thus, the film recorded the Prince twice but during the projection, we have the impression that it is the same moment.
Backwards: in truth, what we see is the instant when the candles are blown out, but at that moment, the film is projected backwards, thereby giving the impression that, on the contrary, they are lighting up. For this scene, Demy took inspiration from a film by Jean Cocteau: Beauty and the Beast.
In slow motion: in truth, the Fairy was filmed descending from the ceiling. It is that shot that is finally projected, but backwards and in slow motion, which gives the impression of the Fairy going up through the ceiling.
Projection: the fabric of the weather-coloured dress is the same as that of cinema screens. Demy filmed clouds in the sky which he then projected on the dress at the moment of shooting the scene. When the Princess moved, she was followed by the machine projecting the clouds, which was mounted on tracks (this is what is called a « dolly » or « tracking » shot). Thus, the clouds are always visible on the dress.
Cinema is magic
Unlike special effects made in the laboratory after filming, for Jacques Demy, « special effects » had a home-made side, « cobbled together » at the moment, using the possibilities of film. When the Prince escapes from his bed, the filmmaker uses the process of double exposure. For the candles that flare up by themselves, he runs the film backwards. He also makes the Fairy fly up towards the ceiling in slow motion. And he uses the projection of moving images to create the effect of clouds on the weather-coloured dress.