This documentation is not using the current rendering mechanism and is probably outdated. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
There is a section for gallery maintenance in the backend. You can open it via the dropdownlist in the YAG module:
Select ‘Gallery Maintainance’ there and you will see the following screen:
Besides a statistics of galleries, albums and images on your site, there is also some information about cached files and the amount of data used for image files on your harddisk.
The last two links of the site are used for maintaining the cached files. The cache is filled automatically if an image is requested in a specific size and configuration.
When clicking on ‘Clear resolution file cache’, a message informs you, that the cache has been deleted.
You can build all resolutions at once, this speeds up the page creation in frontend for the first user of your site. When you click on ‘Build resolution file cache’, there is a progress bar, that shows you the progress of rebuilding the cache: