Appears to be something to do with Batch Distillation where its distilled into chambers, before continuously being recharged.
http://en.wikipedia.org/wiki/Batch_distillationThe information below is quoted from here, possibly where you got that photo from -
http://www.oxygenee.com/absinthe/alambics1.html"The rectifying balls return some of the heavier vapours to the pot, while the balance condenses in the cooling coils submerged in water in
the grey coloured tank below, before being collected in the horizontally mounted cylindrical distillate tanks at the bottom".
"It's possible to conduct two distillations simultaneously with this apparatus".
More information, photos and schematics can be found here -
http://www.oxygenee.com/absinthe/alambics2.html