I had always had the greatest respect for the engineering skills of the admirable Terrot concern - until I saw this patent. It appears that Terrot, of all people, were responsible for the accursed groove that French freewheel manufacturers insisted on cutting into the tops of the teeth on their freewheels right up to the point, in the late 20th century, at which they were (thankfully) finally declared bankrupt or sold to SRAM for one centime.
Terrot, like Atom and Maillard after them, claim that this groove aids shifting. I have to say that I have never found this to be true. My experience is that the groove provides the perfect ledge for the chain to sit on, perched halfway between one gear and another, while your legs slip crazily, the crank spins at hypersonic speed and the pedal’s plethora of tiny chromed teeth are hammered deep into the soft tissue at the back of your calf.