What Flux does is allow the game master to swap campaigns in the middle of a game. The characters are going through their normal adventure / campaign, when the game master cracks open a new set of books, hands over new character sheets, and just... continues. The character sheets are rough parallels to the current characters the players were using, adapted to the new game world. The characters also have some memories from the previous campaign world, and can even try to 'break' the rules of reality by drawing on their abilities / powers / knowledge from the prior world. (Reality gets a bit pissy if they do this though. Just saying.) The game master may do this further down the road, and bring in a third game, and the characters swap over again. Each time they swap, the earlier and earlier campaigns become harder to remember and draw upon, but the characters can -- as well as any other intermediate worlds they once existed in.
The interesting thing is that the game master makes the character sheets (though it is possible for exceptions to occur, and one awesome type of exception as well), building them on the core concepts of the current characters. In addition, NPCs from the setting also cross over, but unlike the PCs, they usually have no memories of prior worlds. And while these NPCs may have similarities to their prior selves (keeping core concepts), there can be other, interesting, twists on their relationships with the PCs. This allows the game master to tweak the campaign while also adding the nice switch to the plot and to the setting, keeping things fresh while holding to (more or less) the same narrative as before.
So, in my current nWoD campaign, I have a "fish" changeling who is currently Queen of Autumn, and who has a house filled with changelings recently rescued from Arcadia (and many still suffering shock). I have a werewolf alpha, who is essentially an uber-cheerleader and leader of the group, and I have a German DJ Mage who is rather disorganized and eccentric.
I decided originally to flux this into 7th Sea, but found that to be a bit more math than I was willing to put into it. I love 7th Sea, but it is more a game I'd rather play, than GM. So instead, I'm using the Qin RPG and using my own campaign world (Paths of Xia). This allows me to go into the martial arts wuxia sort of genre, while fleshing out a campaign world I plan on actually publishing. Character creation has been a breeze, and soon we'll have:
A pirate captain from the north, with her really eccentric crew. A dancer / exorcist from the Court of Steel (a region full of demons and monsters), sent to the heart of the empire to find the person who murdered the Emperor, and a somewhat unorthodox monk with great skill at alchemy. I've kept tabs on the core elements these players have kept bringing up with their characters, and such things have survived the transition, and I'm looking forward to the shift in NPCs as well.
This is going to be so much fun. And the players seem quite enthusiastic about this, too.
Caveat: The rules and system presented for Flux attaches very nicely to any game you want to run. Seriously, it's very unobtrusive. However, the system also presumes you'll follow (at least somewhat) John's style of GMing as well. In essence, control of balance and the campaign is the task of the game master, and not the game engine itself. From what I read, this isn't going to affect my campaign at all -- I'm quite accustomed to players have some seriously "game-breaking" stuff, and have not had it negatively impact my campaign a whit. Some game masters may have trouble with the idea of swapping from a Mage campaign to a Shadowrun campaign, and having people intermittantly performing Mage-Level Sorcery, or having a D&D campaign swap to Vampire the Requiem, and have a Ventrue decide to Turn Undead on the Elysium. There is a single 'check' against this in the mechanics, and one I find amusing and appropriate, but I don't think this is for everyone. It should be (in my opinion), but probably isn't. :)