Since 11g, you can turn Flashback Database on and off without having to restart the Instance. It can stay in status OPEN, whereas in 10g, you had to go in Status MOUNT to change the Flashback mode, similar as it is still necessary to go to MOUNT in order to change the Archivelog mode.

Here is a short demo on the command line:

SQL> select * from v$version;

BANNER

--------------------------------------------------------------------------------

Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production

PL/SQL Release 11.2.0.1.0 - Production

CORE    11.2.0.1.0      Production

TNS for Linux: Version 11.2.0.1.0 - Production

NLSRTL Version 11.2.0.1.0 - Production

SQL> select log_mode,flashback_on from v$database;

LOG_MODE     FLASHBACK_ON

------------ ------------------

ARCHIVELOG   NO

SQL> select status from v$instance;

STATUS

------------

OPEN

I am already in Archivelog mode, but Flashback (Database) is not turned on yet. Instance is OPEN – and stays there:

SQL> alter database flashback on;

Database altered.

SQL> select log_mode,flashback_on from v$database;

LOG_MODE     FLASHBACK_ON

------------ ------------------

ARCHIVELOG   YES

SQL> alter database flashback off;

Database altered.

SQL> select log_mode,flashback_on from v$database;

LOG_MODE     FLASHBACK_ON

------------ ------------------

ARCHIVELOG   NO

Conclusion: If you are about to do a critical operation with your database that you may want to rewind in case, you can turn on Flashback Logging before that operation and turn it off afterwards without limiting the availability of your Database!

Watch me on YouTube explaining the above:

Keep in mind that this affects only your ability to do FLASHBACK DATABASE, not the other forms of Flashback like FLASHBACK QUERY or FLASHBACK TABLE because they do not need Flashback Logs and work regardless of what you see in the column FLASHBACK_ON from V$DATABASE as explained here.