Each forum (or tag, if you will) has an "add forum" or "remove forum" at the top right in a green link, that is how you can manually override what happens automatically.
What happens automatically, is that each time you vote on a topic, that data is aggregated to the topic tags/forums, so each user has a running score of what forums they tend to like and what they tend to dislike.
For new users and some use patterns there is not much data for the automatic part and not enough forums added at the beginning (which is an easy change we will make in the rewrite, making a default data set to start with) but you can basically go to each forum and pick what you want/don't want included in your filters. The forums you add, and topics you vote up are included, the forums you remove (or that are auto removed based on vote history) are the ones excluded.
There might also be a popularity bust-through figured in to the algorithm, so that it only filters low scoring topics but it's been years since we wrote that and I don't recall the exact details right now.