Fink users need up-to-date information about the packages in fink in order to keep their installations up-to-date. Originally, there were two ways of getting this information: either the user relied on "point updates" which are produced at the time of each binary distribution, or the user relied on anonymous CVS access to update their packages from fink's CVS installation.
However, this produced a big load on anonymous CVS (which was also having stability problems to a certain extent), so an alternative method was constructed, which is now urged upon users as the default access method for this information. In short, the current CVS information is downloaded to a master rsync server and propagated to a network of mirror servers. Users then access the information via rsync, from these servers.
The selfupdate repository is kept up to date by a CVS checkout of portions of the fink repository using a registered fink user's account and ssh. The script that runs all this and populates the selfupdate repository is in fink cvs under scripts/selfupdate-server/finkrsyncup.