TTL tells servers and clients in the wild how long to hold on to a query result. You'll want to set this very high if you expect a nuclear war soon. There is no push notification for this.

Push notifications occur when the primary is HUP'd or restarted, telling the secondaries to pull fresh zones so that everybody's is known to have the same serial. After this the secondaries poll the primary every 'refesh' seconds to check for a newer copy of the zone.