Anthony Bouchard, writing for iDownloadBlog:
To access Emergency SOS, all you need to do is press and hold the side button on your Apple Watch, and you’ll see a new red “SOS” slider at the bottom of the slider list. Sliding this slider initiates a 911 call and fires off text messages to your emergency contacts within seconds.
But there’s more to it than that. Read the article and learn about auto-call and how to set up your emergency contacts.
One of the few problems I regularly ran into with watchOS 1 & 2 was it thinking I was holding down the side button seconds after I just tapped it. Totally software. So I don’t think I’d ever trust this feature, even if I needed it.