Apple Watch Emergency Button
In watchOS 3.0 users can press and hold the side button on the Apple Watch to quickly start an emergency call.
When in the U.S. this will call 911, but it also works worldwide and it knows which number to call depending on your where you are.
The 911 call can go over cellular or on WiFi if your phone is not working and the Apple Watch has a WiFi connection. You can talk to emergency responders directly from your Apple Watch and then it will show your Emergency ID, which is another new feature.
It will also send an SOS to select friends and family with your location included in the alert.