If you look at your bike's cassette in the rear, you'll see a similar thing. You will also see ramps cut into the side of the cogs or rings. If you put the bike up into a stand and turn the pedals very slowly while shifting the gears, you will notice the chain only shifts at these ramped portions. Not only that, the chain can physically engage both the cog or chainring it is shifting off of, and the cog or chainring it is shifting onto. These features are why drivetrains shift so quietly. It also allows for less finessing; you don't have to "soft pedal" nearly as much when changing gears compared to pre-CAD drivetrains.