What should show as "last Backed up"?

On My Passport, Why does “last backup” on the 2nd day of backup only show 42 MB, when it backed up 20.1 GB the first day?

Hello,

Please be informed that WD Backup does an incremental backup. Every time you disconnect and reconnect a backup target or source device, the WD Backup software rescans the backup source device for new or changed files and resumes automatic backup protection according to the schedule.

Please refer to the following KBA article: How To Backup Files Using WD Backup

So if I just leave it plugged in all the time, everytime i turn on the computer and change or add files it will back up those onto what was already there?