MMA Weekly does a decent job of breaking down how they work. To wit:
This opens up avenues for fighter sharing between the ProElite brands, but it isn't a free fighter exchange as each promotion will need to fund the purse of the contracted fighter.
Example in place: Murilo "Ninja" Rua is an EliteXC contracted fighter. Hypothetically, let's assume he has two of his four contracted fights run already, so he signs to fight Xavier Foupa Pokam at Cage Rage. Cage Rage have to foot the bill for Rua to fight on their show, but my understanding of the situation is that the fight does not count as part of his contracted fights list, meaning EliteXC still has access to his services for the remainder of his contract. But because he is fighting within the network, the fighter sharing ethos works without damaging a fighters perceived value to a specific promotion.
This approach opens up opportunities for fighters to take new challenges and vie for belts in other promotions, as well as maintain an active fight career during times when a card is fully stacked in his `native' promotion.
I guess that still leaves the question of time tables. If you're Ninja and you still have two fights left on your EliteXC contract, is there a time limit regarding when those last two fights must be satisfied? You can't fight for other promotions forever, can you?