Here's a vid: https://www.youtube.com/watch?v=YeqvTJ-Y22Y
As the guy notes, these are and have been commerciably available for a long time, but usually one piece to link two specific parts together. This one will basically do whatever you want it to (barring some exceptions where the ratios are too far apart).
I wouldn't go world touring on it, it seems liable to break or gunk up and you'll probably increase your consumption of cables, but for throwing together the parts bin special this seems pretty good.
Basically just converts pull ratios via 2 differently sized wheels. Makes it so one click of the shifter moves the cable by, say, 2mm instead of 1mm.