Let me quote Oracle documentation...
"Incremental Backup Algorithm --
Each data block in a datafile contains a system change number (SCN), which is the SCN at which the most recent change was made to the block. During an incremental backup, RMAN reads the SCN of each data block in the input file and compares it to the checkpoint SCN of the parent incremental backup. (If block change tracking is enabled, RMAN does not read the portions of the file known to have not changed since the parent incremental backup.) If the SCN in the input data block is greater than or equal to the checkpoint SCN of the parent, then RMAN copies the block.
One consequence of this mechanism is that RMAN applies all blocks containing changed data during recovery—even if the change is to an object created with the NOLOGGING option. Hence, making incremental backups is a safeguard against the loss of changes made by NOLOGGING operations." (1)
(1) RMAN Backup Types -- Oracle® Database Backup and Recovery Advanced User's Guide
10g Release 2 (10.2)
Part Number B14191-02