There's a Reddit thread between various users and keltranis, (one of the more senior coders Reddit ever had) explaining the code here:

http://www.reddit.com/r/programming/comments/td4tz/reddits_a...

And a quotation for those not wanting to click:

  *ProfDrMorph* 2 points 1 year ago
  So that means all posts in all subreddits (when browsing
  'hot') are sorted this way:
  1. all posts with more upvotes than downvotes with the
  order determined by age (newer posts are preferred) and
  popularity
  2. all posts with the same number of up- and downvotes in
  whatever order the database returns them
  3. all posts with less upvotes than downvotes with the
  order determined by age (older posts are preferred) and
  popularity (posts with a lot more downvotes are preferred)
  Because that's what the _hot() function implies if the
  sorting algorithm uses it as a 'key'.

  *ketralnis* 2 points 1 year ago
  Yes that's accurate