It listens to the "selection-pastebin:remote-pastebin" action, which, when activated, runs a command with "%" replaced by the name of the textfile. This command can be set via a resource:
URxvt.selection-pastebin.cmd: rsync -apP % ruth:/var/www/www.ta-sa.org/files/txt/.
And the default is likely not useful to anybody but the few people around here :)
The name of the textfile is the hex encoded md5 sum of the selection, so the same content should lead to the same filename.
After a successful upload the selection will be replaced by the text given in the "selection-pastebin-url" resource (again, the % is the placeholder for the filename):
Note to xrdb users: xrdb uses the C preprocessor, which might interpret the double "/" characters as comment start. Use "\057\057" instead, which works regardless of whether xrdb is used to parse the resource file or not.