There are fifteen fandoms that you can cross SPN with. You tell us what fandoms you can write (at least two of the fifteen) and make up to four requests for crossover stories you'd like to have written for you (at least two different fandoms in the requests). We assign you someone to write a story for and we assign someone to write a story for you.
Then, on August 18, there is a lot of awesome SPN crossover fic posted!