The Get bracelet uses bone conduction for audio, as well as an accelerometer for hand-movement gestures. You hear the audio of phone calls, notifications and other sounds by literally sticking your finger in your ear. The bone conduction conducts the noise from your wrist bone to your finger bone and into your ear bone, or whatever.
You can answer the phone by doing an answer-the-phone gesture with your hand and arm. And you can control other features with other gestures.
I’m probably in the minority, but I’d love to see a stereo version involving two smart bands, two fingers and both ears.