The information is not copied by the primary, but instead is pulled by the secondaries. The rules however, are set by the primary (see the section about secondaries requirements in the primary specification screen).
Clearly, the secondary DNS administrator has to tell his DNS to act as a secondary for a given domain, but the primary DNS will tell it how!
The secondary will regularly talk (as told by the primary) to the primary and check the serial number (revision number) of the domain. If the serial number of the primary is the same as the copy of the secondary, or is smaller, no copy will be made.
A common error is to forget to update the serial number (SOA record). Secondaries won't copy the new information. This may lead to strange behavior. Linuxconf manages serial numbers automatically!