Yeah - good luck with figuring that out!

From here: http://bikehugger.com/2009/03/mystery-portuguese...
And here: http://bttdominhoaovouga.blogspot.com/ Spoilers in the comments - don't read this if you want to guess what it is

P.S. My guess is the hub has a sliding mechanism inside whereby it's driven by either the large cog or small cog, providing two speed shifting at the rear (and two at the front, for four speed overall).