What it also does, regardless of whether you tap the SOS button, is lock down your device and disable Touch ID until you verify your identity with your passcode. A new setting, created to automate emergency services calls, lets iPhone users tap the power button quickly five times to call 911.
Someone might be interested in keeping a criminal from using your fingerprint to force your device open, too.
This new setting (being called a "cop button") is now in beta and seems to be designed as a way to automate emergency service calls, as well.
As pointed out in a report by The Verge, some users have discovered that when you quickly tap the power button five times on a device that is now running iOS 11 public beta, the option to call emergency call numbers is presented as usual but the system automatically disables Touch ID as well. The Verge first reported this "iOS 11 cop button", which appears to be a super quick way to disable Touch ID when you think you might be physically forced to unlock your smartphone. In fact, while Touch ID might be more convenient than a PIN code or password, there are cases when it could be less resilient from legal compulsion to unlock your device. The new method presented by the iOS 11 Emergency SOS feature is both quicker and more discreet.
Previously, if you wanted to turn off Touch ID you had to go through a long and complicated process. This means that the users are required to enter the passcode before they can start using Touch ID again.
These situations may be limited, but the feature will prevent other people from easily accessing their iPhone by pressing their finger on the device.