Well , here is some more explaination ...as per the oracle documentation says , the before and after images are witten to the redo logs .once the redo logs have been applied in recovery process , and this means recovering the rollback segment entries in the buffer cache that may have been lost in a database crash -the rollback segments are applied-but the UNDO informati0on comes from the redo logs firtst .So the redo logs contains both undo and redo information .
Hope that doesn's confuse .