The SERVERS in the zone recognize that the data collector has gone down and start the election process. In this example the back up data collector is elected as the new data collector for the zone.
The member servers in the zone then send all of their INFORMATION to the new data collector for the zone. This is a function of the number each server has of sessions, disconnected session, and applications.
In TURN, the new data collector replicates this information to all other data collectors in the farm. Even if a Data collector is unavailable the servers will CONTINUE to function. The users that are already logged in will not be affected.