It manages the feat by checking for exact DNA matches between you and over 130,000 people with known ancestry in those 120 new regions. If you match at least five people from a region, that's deemed to be one of your recent ancestor locations.The company will also tell you the strength of a match based on the amount of DNA you share with people from a given area.
This will help right away if you're taking the 23andMe test for the first time, although it'll roll out to existing customers over the "coming months." There are also some tweaks to accompany the more comprehensive data. The company has relabeled some regions for better accuracy ("Yakut" is now "Siberian," for instance), and has improved the map to help you zoom into a given region. You might not get any mind-blowing revelations about your genes -- this can only be so precise -- but it won't hurt to come back and see if there are any new insights.