I think what Hrishy has said is totaly correct. He haven't said that db blocks are not written into db files. He only pointed out that during backup period whole db blocks are written into *redo logs*, hence the amount of redo logs filled during backup can be much higher than during normal operation.
Originally posted by tamilselvan
Hrishy, You are wrong. When the tablespace is in BACKUP mode, only the header is freezed, all transaction data is written into respective data block on the data files.