Answered in the FAQs, but in short; random chance (or the PHP rand() function). When the number of posts are good enough to make it worth publishing, then a ra

Answered in the FAQs, but in short; random chance (or the PHP rand() function). When the number of posts are good enough to make it worth publishing, then a random post will be picked, and that user will get sent a "You're up! Make any final adjustments now" email. The only manual intervention will be to ensure that posts meet the posting guidelines (i.e. aren't spam, and aren't malicious).