Imagine this: you're on your way through the parking lot from the mall after a long day of holiday shopping; you suddenly feel the presence of someone following you.
Your concern is great enough that you want to call 911. In order to make that call you must take out your cellphone, unlock the screen, find the phone app and dial the numbers.
There's good news in a situation like that one: Apple includes a new feature in iOS 11 that might save your life.
To quickly dial 911 you only need to press the on/off button, the one on the right side of the phone, 5 times quickly. The action will sound an alarm and begin to dial the nearest 911 call center based on your phone's GPS locator. The feature works even if you have location settings turned off.
It's called "Emergency SOS" and you'll find settings for it on the main screen of the settings app.
Apple has more information on the tool here https://support.apple.com/en-us/HT208076
Some iPhones and earlier versions of iOS may require you to slide an emergency icon on the home screen to make the call but new iPhones, iPhone 8, 8 plus and X, you only need to press the button 5 times.
Once the call to 911 goes out the phone will send a text message to anyone on your emergency contact list which states you've called for help and will show your exact location. It also will send those emergency contacts follow-up text messages if your location changes. If the button is pressed 5 times accidentally there are a few seconds before the call goes out.
This is a great tool in emergencies as you can call for help even with the phone in your pocket. In case you haven't done it already, this is a good time to set up your emergency contact list and your medical ID. This can be set using the iPhone's Health app and by going to 'settings', 'emergency SOS' and follow the directions to the Health app.
It is a feature I hope you never need but for safety's sake, it's nice to know it's there.