Well... To start all changes are made in memory and not to disk, and the changes to the files on disk are done later (by a background process) at a page level, not a row level...

As far as I can tell, updating a row and not actually changing the values does not mark the page as dirty, meaning it won't be written back to disk when the checkpoint process runs.

I'm not saying it isn't ever marked dirty, but in my simple tests with a single row in a table, updating the name to itself did not appear to mark the page as dirty.