--modify-window=1: this is ESSENTIAL. Basically in windows filesystems time is kept in even numbers (or some such problem). This command tells rsync to ignore filechanges that are only 1 second in difference from the original. It is almost impossible that you will create a file, sync it, and in ONE second make a change and want to sync it again. So it is safe to use this option and it means that rsync will not back up everything every time simply because of a one second change.

via: http://ubuntuforums.org/showthread.php?t=820425