Hello,
Disabling the "Scan when a file is backed up" would not scans the files when they are accessed during a backup operation.
Use this option if you did not run a virus check on the files that you want to back up.
Do not enable this option if you want to bypass Auto-Protect for the files that the client backs up. By using this option, you can significantly slow backup operations, because Auto-Protect scans each file that is included in the backup.
The setting applies only to files that are backed up. The client scans the files that it restores from a backup regardless of this setting.
You can lock or unlock this option to prevent or allow user changes.
Reference: https://www-secure.symantec.com/connect/forums/scan-when-file-backed
Hope that helps!!