Will Collins, it's massively good fun.
You sign up to give and receive (or just give) a story starting on 1 December.* Cobra and his elves collect all the prompts, use arcane magic to decide who is getting which prompt and then the prompts are given out on Promptmas (aka Christmas Day). You then have a couple of months to write a story for your giftee. During that time there is much fun and encouraging in the thread. It's a secret who you're writing for, no one knows but you and Cobra (and your beta if you have one). Once the stories are given out, the guessing thread goes up, where a snippet from each story is posted, and everyone tries to guess who wrote what.
It's the most fun you can have with your clothes on!
All that said... Are we there yet? Are we there yet?
*The new season of sisyfus will begin in a new thread, not this one, so keep an eye out for it--it'll be titled the same as this one, only with 2020 in the title.