CDA explained the rules thusly:
“We start out with a map of the world in OTL . Then each person creates another map, showing the world 1-20 years in the future, after some major border changes. Then the next person shows the world 1-20 years after that, and so on. The intervals must be 20 years or less. [When we reach the 20th Century we change the intervals to 1 - 5 years, ending at the Millennium.]
The catch is that the people don't post anything other than the map and the year, and don't actually decide on their turn what events led to those changes, or even what the new countries are (but try to use UCS). Once we get to the present day, we go back and outline exactly what happened.”
You can find this map game's thread here.