Sorry about the incomplete Codex. We are populating it as fast as Brandon can write!
If you look at the three main views (not using iForum theme but say, default theme) then each one populates a data object. The first populates the groups and the forum rows. the second the selected forum and topic rows and the third the selected topic and post rows. So the forum data object ($spForumView) only has that forum record and related topic data (to fill the current page). That also generates the $spThisTopic data object which points to the current topic being displayed.
Calling sp_this_topic() will populate the $spTopicView data object and it’s collection – $spThisTopic. This has to be done before the loop is started. To pass the topic ID just use $spThisTopic->topic_id.
If you go to the forum admin > Toolbox > Data Inspector you can actually display all of these data objects to the screen when you display the relevant page which is very useful to see what data is in each collection object.
But I still don’t know for sure what will happen when you try this. I am actually both horrified and fascinated 