I figured out that Rublog does not update the sidebar, or much else for that matter until you drop a new post into its directory. So, without looking at the code much, I think what is going on is that someplace, there is a last update time for the blog, and when someone requests the page, the blog looks at the mod times on all of the files in the post directory and then compares those to the last blog update time. If there is a new post, it updates the page.
I am trying to figure out how to add simple text to the sidebar, and also, how to add something to the bottom of the page.