Specific question on Stack Overflow: https://stackoverflow.com/questions/15468735/overview-of-how-mongodb-uses-its-various-threads
In short, the OP is asking about what MongoDB uses it's threads for and how to tune this. I was conflicted about referring the OP to Server Fault or here for related questions.
Part of the question related to the first point on the faq, but questions about how a particular database works wasn't mentioned at all on the faq.
So, are questions about how a database is implemented on topic here?