Enabling --log-update

I need to track changes to my database while I'm doing a pg_dump.

I've found a reference to do this
http://www.arkeia.com/hotbackup/postgresql/ (see the second method).

It states that I need to put my database in the "mode --log-update"

I've tried to do this, but I keep getting an error saying that
--log-update needs an argument.

I can't find any documentation on this switch. Please help!

If I can't get this working my next alternative is to use either
http://gborg.postgresql.org/project/tablelog/projdisplay.php or
http://gborg.postgresql.org/project/audittrail/projdisplay.php

I need to have the database available while the dump is done.

Thanks is advance

Dan Hrabarchuk