Very simply, it adds a "Content ID from Alias" to the list of options for supplying a default value to a contextual filter when the value is not present in the URL.
It works by getting the request path, and checking if the path translates successfully to a node system path. If it does it returns the node ID, otherwise it continues down the path argument chain, chopping the right-most path argument off and checking the remainder. This repeats until no path arguments remain, at which point false will be returned.
Relased under the GNU General Public License v2.