It's pretty simple. Can't remember who made the first concurrent game, but ChrTh (old-timer here) has created one as well I think, and the idea seems to be gaining popularity.
It onvolves more play and basically works like this. The first team of three all play 30 turns from 4000BC, and post the saves. We all decide which is the best, and team 2 all play from that save. Team 1 then plays from Team 2's best save and so on.
It makes for much more discussion, deciding which is the best save (usually comes to a vote), and is also good for learning because it's easier to see where you went wrong compared to the other saves (assuming yours isn't the best).