iOS 11’s Emergency SOS Feature
Emergency SOS is activated by pressing on the sleep/wake button of an iPhone five times in rapid succession. When the requisite number of presses is complete, it brings up a screen that offers buttons to power off the iPhone, bring up your Medical ID (if filled out) and make an emergency 911 call.
Along with these options, there’s also a cancel button. If you hit the sleep/wake button five times and then hit cancel, it disables Touch ID and requires a passcode before Touch ID can be re-enabled. Touch ID is also disabled if you actually make an emergency call.
I presume this is so that people can’t get into your phone after you call 911 and pass out. However, it’s also a way to temporarily disable Touch ID, so you can’t be forced to unlock your phone.
