First, in the GUI, click on Help -> Help Topics, and search for Enhanced Incremental. This is going to give you a lot of information, including how it is used
In an existing Backup Specification (or a new one), in the Options tab, click on the Advanced button in the File System Options. Click on the Other tab, and you will see an entry for Enhanced Incremental. Click on the Check box to enable this, click OK and Apply
Please see the attachment for an example