Update Slave from a Dynamic DNS of Master

[Lists User]

Might be quick question... I might be lost...

When I update the Master DNS by nsupdate it isn't updating the
slave... The serial number is never updated so I don't know how
the slave would ever realize to get the IXFR.

What am I missing here?  What tells the master to send the update
to the slave also?
Also vice versa.  If I was updating the slave, how would it know
to update the master?

Thanks for any help,
-Steve