This is the way the owner of my LBS stated it when I asked the question when I just started biking again a couple of years ago. Think of the cassette as a mountain or hill on it's side, where the largest cog is the bottom(lowest part) of the mountain/hill and the smallest cog is at the top(high part) of the mountain/hill--bottom of the cassette hill, low gear, top of the cassette hill, high gear.