How does the client DNS update referral mechanism work?

Because the DNS server that runs on an RODC cannot directly register client updates, it has to refer the client to a DNS server that hosts a primary or Active Directory-integrated copy of the zone file. This server is sometimes referred to as a “writable DNS server.” When a client presents a Find Authoritative Query, which is the precursor to an update request, the DNS server on the RODC uses the domain controller Locator to find domain controllers in the closest site.
The RODC then compares the list of domain controllers that is returned with the list of name server (NS) resource records that it has. The RODC returns to the client the NS resource record of a writable DNS server that the client can use to perform the update. The client can then perform its update.
If no domain controller in the closest site matches an entry in the list of NS records for the zone, the RODC attempts to discover any domain controller in the forest that matches an entry in the list.
Suppose that a new client is introduced to a site that has a DNS server running only on an RODC. In this case, the RODC DNS server tries to replicate the DNS record that the client has tried to update on the writable DNS server. This occurs approximately five minutes after the RODC provides a response to the original Find Authoritative Query.
If the DNS client on the RODC attempts a DNS update, a writable domain controller running Windows Server 2008 is returned so that the RODC can perform the update.