Share Your Heartbeat
The Apple Watch can track your heart rate for fitness activities, but that's not all the sensor can do. You can share your heartbeat with a friend.
When you do this a small animation shows on-screen to show your heartbeat directly to another Apple watch. Apple calls this, "a simple intimate way to tell someone how you feel."
After using the Apple Watch for over a year, this is a feature we don't use that much, but it's a cool way to send a message to someone you love to let them know you are thinking about them.