When an application tries to access virtual directory in virtual base directory, Autodir creates virtual directory in it if it does not exist already and mounts the real directory on it from real base directory. But once this happens and if this virtual directory is not accessed from virtual base directory for a specified time period by any application, this directory is removed and accordingly that corresponding real directory in real base directory is marked for backup.
The time period to wait for expiration can be given through command line option to Autodir.