The idea is that you only back up anything that's changed, so the backup is quicker.
backup incremental level 1 database
And then you subsequently 'refresh' that image copy by (basically) recovering it:
backup tag 'MYDB' for recover of copy with tag 'MYDB' database plus archivelog delete input;
(I'm doing the commands from memory, so I apologise if they can't exactly be cut-and-pasted).
recover copy of database with tag 'MYDB';