You post an anonymous prompt for characters and a desired kink. Which, in the usual fannish sense, could be anything from schmoopy flufic to conventional sexual kinks. Somebody sees your prompt, likes it, and writes an anonymous response. Short, usually, but not always. In turn, you scan the list of prompts and fulfill a request yourself, thus earning good karma.
Great way to get your feet wet, I think. No pressure, no obligations, no judgement. Just fun.
Speaking of making things happen oneself, I also need to get myself into gear on the Buffyverse Big Bang aka buffy_bigbang. sahiya has approved my schedule and rules proposal, so I need to write it up for real with all the details I can pack in. And then post it.