Backup Past 24 Hours Only

I have an application that dumps its backup files to a network location.

I want to run a single backup job that copies to tape only what has changed in the past 24 hours.

I know i can do a Full then an incremental.  but this means i have to run a full once a week.

Using BE 2010 there is a backup method called Working Set - Changed today.

What does Changed today mean?  past 24 hours from the last backup or from 00:01 to the time of the job?

What is my best option to just backup the changed files for the last 24 hours or last backup job?