If your project is sufficiently complex, you'll discover that it's sometimes nice to forcibly “backdate” portions of your working copy to an earlier revision; you'll learn how to do that in Chapter 3. Perhaps you'd like to test an earlier version of a sub-module contained in a subdirectory, or perhaps you'd like to figure out when a bug first came into existence in a specific file. This is the “time machine” aspect of a version control system — the feature which allows you to move any portion of your working copy forward and backward in history.