This January 1952 patent describes the Huret Spécial Lousion Bobet. Huret took the, very satisfactory, Huret Compétition and 'improved' it by adding a second cable that controlled the chain tension. This allowed the rider to reduce the chain tension while changing gear.
This issue of chain tension while changing gear was a 1950s obsession. It was not just Huret: Super Champion, Simplex and the Cyclo Gear Company also had models at one time or another with this feature. (As an aside there is even a 1972 Czech patent showing this feature added to a Favorit Special parallelogram derailleur). I have briefly ridden a number of bikes with chain tension controls on their derailleurs and have never found it to improve the gear change at all - perhaps I am missing something.