The Taptic Engine—and the shaking weight that composes it—is one of the darkest, and therefore densest components in the watch.

In order to get the most bang (vibration) for its buck (space), Apple packed a tiny, heavy weight into its Taptic Engine to provide the vibratory feedback to your wrist.

Thanks to the X-ray, you can see the "engine" is a linear actuator. The springs move the weight up and down (side to side, when on your wrist) to create the vibration effect.

Creative Electron has a great video of some vibration motors running in real time—inside an x-ray machine. Rad.