This feature works very similarly to the discover and import archive feature. The main difference is that whereas the import archive feature is used to import archives stored on your server, the Import archives from S3 feature is used to import archives stored in the Amazon S3 storage service, even if you haven't used the application to store them there.
Start by entering your Access Key and Secret Key in the fields and click on the Connect to S3 button. The page reloads and you see a new drop-down listing all your S3 buckets. Select a bucket and click on the button. You now see a list of subdirectories and files in your bucket. Just navigate inside the directory containing your backup archives and select the backup archive you want to import. That's it!