archive_command

    I've been playing around with recovery and am a bit confused as to
when this method gets invoked.
    At the moment, I have two servers set up, both of them with postgres
installed.  Lets call them server A and server B.  Server A is running
postgres and processing transactions.  Server B has postgres installed,
but the postmaster is not running.  My archive_command is set to use
rsync copy the wal log from server A to server B.  When i'm watching the
number of wal files on server A I notice sometimes there are 3 files
that have not been archived, sometimes there are 4, and sometimes there
are more.
    So, my question is, when is archive_command invoked?

Thanks in advance!
Kris