Here Bruce describes the current algorithm for determining if a host is "slow". If it is, it will be given a "short datafile" (i.e. a datafile that corresponds to short workunits) when the host needs to download a new one. Fast hosts are given datafiles (and thus long or short tasks based on it) simply as they come, more or less randomly from the database.
BM

Long or short WU's?
)
I'll bump up the quota a little bit more...
Bruce