During normal operating mode, only the specific changes are written to the online redo logs. However, when the tablespace is being backed up, it is possible for the database block to be modified while it is being backed up. This means that part of the block will be "new" and part of the block will be "old" in your backup copy. This is called a split block. To help with this split block issue, Oracle writes the entire block (not just the part that was changed) to the online redo logs when the tablespace is in BACKUP mode.
This was first published in February 2007