Hi Jaswant,
It can all be found in mod/forum/lib.php in the forum_cron() function.
The 'mailnow' setting (which has now been renamed) skips the standard 30 minute wait time. It does not cause the message to be sent 'immediately' as this would create a less-than satisfactory experience for the user writing the message (they'd have to wait for Moodle to work out who to send it to, and send each individual message).
What are you hoping to do with this information? There may be a better way to achieve this if you post your goal rather than the implementation you think you need.
Hope this helps,
Andrew