Emergency SOS feature on an Apple Watch 4 will dial 911 when you are unable to call. When you invoke Emergency SOS, your watch will automatically calls the local emergency number (911).
If you have emergency contacts setup, then this feature will also notify emergency contacts with text message, sends them your current location, and display your Medical ID badge as well for emergency personnel.
Press and hold the slide button ( which is on the right side)
Slide the Emergency SOS to make the call
Apple watch will call your local emergency number (911)
You can also slide Medical ID to see your medical information
If you have cellular version Apple watch 4 then it will does SOS while you are away from your phone.