The algorithm as described in this document works by always looking up filenames in directories (a stat in unix terminology). A good implementation is expected to read the directories once, and do all lookups in memory using that information.
This caching can make it impossible for users to add icons without having to restart applications. In order to handle this any implementation that does caching is required to look at the mtime of the toplevel icon directories when doing a cache lookup, unless it already did so less than 5 seconds ago. This means that any icon editor or theme installation program need only to change the mtime of the the toplevel directory where it changed the theme to make sure that the new icons will eventually get used.