That screen has led to confusion a time or two.
When you first hit the "Hot!" button, the screen you're taken to is the "Active Boards" list. It simply displays the boards with the most recent new posts, in reverse order of date/time.
If you click "Top Boards" while you're there, you'll see a list (compiled in realtime, btw) of boards that're really "hot" in terms of a formula I use that takes into account total posts, activity in the past 30 days, and number of bookmarks.
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