The system works simply with two 1-speed freewheels, one freewheels forward, one rearward. So always, one is engaged while the other freewheels. The chain goes in a figure 8, so if you pedal forward, you get one freewheel (and gear), while the other is freewheeling, backwards the other gear.
It's described in "The Dancing Chain." A friend in France has a bike with the system. Maybe we should feature it in Vintage Bicycle Quarterly - a good complement to the Integral in the current issue, with three chainrings, a front derailleur, but one cog in the back and no rear derailleur...
The weight system is known in France as "Systeme Badoit" - a different friend has an old Peugeot with that one. Interesting to say the least!
All these bikes were, for their time, lightweights, so I think they belong on this list.
Jan Heine, Seattle