A user just has to press and hold the side button underneath the Digital Crown, and the emergency call will countdown from three. The Watch will then automatically call 911, or the equivalent for the country the user is in. The call will go through the paired iPhone, but it'll also be able to call directly from the Apple Watch if the Watch is on a Wi-Fi network.
SOS will also send your location to your emergency contacts, alerting them that you're in need of help. The Watch itself can also display a user's MedicalID, easily displaying a user's name, age, date-of-birth, medical conditions and other medical information.