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 the SCN in the input data block is greater than or equal to the checkpoint SCN of the parent, then RMAN copies the block.
Note that if you enable the block change tracking feature, RMAN can refer to the change tracking file to identify changed blocks in datafiles without scanning the full contents of the datafile. Once enabled, block change tracking does not alter how we take or use incremental backups, other than offering increased performance.