Flashback Reinstantiation

Oracle Flashback Reinstantiation:
In an Oracle9i Data Guard environment, a failover operation leads to a resetlogs. This operation invalidates the old primary database. Therefore, you need to perform a hot backup on the new primary database immediately, and then re-create a new standby database. This operation can take a long time, and your new primary database is vulnerable during this period.

The new Flashback Reinstantiation feature reduces the need to reinstantiate the old primary database following a failover. This feature allows you to quickly restore full resiliency after a failure. This is accomplished by using the SQL statement FLASHBACK DATABASE to roll back the old primary database in time to synchronize with the old standby database.
http://oracledocaccess.blogspot.com/2009/04/oracle-flashback.html

A beautiful document covering most of Flash back features of Oracle database.
http://www.orafaq.com/node/31