"Mop up" setting

What exactly does this do? Does a pitcher set to mop up pitch in games when you have a big lead, or when you are getting blown out, or both? Any information about how you have used this setting successfully would be appreciated.
http://forum-365.strat-o-matic.com/archivesn/
http://forum-365.strat-o-matic.com/archivesn/viewtopic.php?f=12&t=442588