Downloading all emails from your Gmail Account every time you want to backup your emails would be a nightmare. In this case it comes in hand the incremental downloading feature of email.
Gmail Backup checks all emails in the Gmail Account whether their are already downloaded. If so than it skips them. It only download emails which are not presented in your local copy. However, the checking emails takes time too.
As result, we recommend to specify the date since you want to download emails. For example:
In the example, I specify that I want do check and eventually download emails since 1 January 2008. Consequently, the process of updating of your backup is much faster.
In a future release, we will add feature which will get, from the local backup, the latest date of your downloaded email. And it will use it for incremental backup. Nonetheless, you must be aware that this solution will not be bullet proof. In some cases, you can get outdated emails resp. new emails with old date.
Your GMail Backup Team