http://ktown.kde.org/~ossi/files/kdm/README appears to be the home site for the README with the tantalizingly incomplete description of "reserve" and FIFO's. Judging from searching KDE's site, this is the only description of the reserve functionality. There is also a kdm handbook at http://docs.kde.org/en/3.2/kdebase/kdm/. As far as I can tell, it is not available in Debian's Khelpcenter, and it has no discussion of "reserve".