In this patent Audouard describe their signature system with a fixed derailleur, and a four speed cog that is shifted laterally to effect the gear change. The lateral shifting is achieved using a well integrated design using two cable-controlled toggle chains inside a hollow hub axle. Both cables are controlled by a single gear lever.
The derailleur itself is a rather sophisticated design with a long-arm, offset, pulley cage holding two lightly toothed pulleys. It’s all remarkably similar to the later, more developed versions of the British TriVelox - but they did not appear for another 25 years.