As suggested by Gerald, snapshot client guide should give you the information that you are looking for..
Additionally, to give a little more info, here is how flashbackups work:
-> FlashBackup creates a map of the file system.
-> NetBackup begins writing the entire raw partition to tape at the block level.
-> A “copy on write” is performed on all mapped data that is written during the backup. Before a block of data is modified, it is copied to a cache file.
-> When the backup process encounters a changed block, the data is read from the cache file. This ensures a consistent point-in-time backup.
-> When data needs to be restored, the administrator has two options- restore any individual file or directory, or restore the entire raw partition.
