Database Backup and Recovery User's Guide says 'The autobackup occurs whenever a backup record is added. If the database runs in ARCHIVELOG mode, then an autobackup is also taken whenever the database structure metadata in the control file changes"
Out of curiosity to see how exactly it works, just added a tablespace and checked my control file autobackup location to see the auto generated backup of controlfile. When i did 'ls' it was not showing any recent controlfile autobackups.
I rechecked all my configuration to make sure all setup is done right. After about 10 minutes i listed again autobackup location, now i can see a new autobackup file is generated.
I spent sometime searching on google to see how long it takes and if the time it takes is configurable. I did not get much on its time lag .
Spending sometime on MOS searching for auto controlfile backup generation time gave me the answer to my question. With 11g 2 control file autobackup is implemented in a differed mode
RMAN CONTROLFILE AUTOBACKUP NOT CREATED DURING DB STRUCTURE CHANGE [ID 1068182.1]