It only works for ambiences that you set as favorites. Go to the Gallery, choose the ambience category. Tap on an ambience. It'll appear magnifed and there'll be a star-icon on the lower right. Tap on this star to make the ambience your favorite. Swipe from right to left to confirm/save. Repeat with other ambiences. Now you can select one of your favorites from the gesture you saw on the video. Took me a while to figure that out. What I don't get is why some ambiences, when set as favorites, can have their own ringtones etc. assigned, while others only let you change the volume for the ambience.