https://www.theverge.com/2017/8/17/16161758/ios-11-touch-id-disable-emergency-services-lock

Apple is adding an easy way to quickly disable Touch ID in iOS 11. A new setting, designed to automate emergency services calls, lets iPhone users tap the power button quickly five times to call 911. This doesn’t automatically dial the emergency services by default, but it brings up the option to and also temporarily disables Touch ID until you enter a passcode. Twitter users discovered the new option in the iOS 11 public beta, and The Verge has verified it works as intended.

Previously, if you wanted to temporarily disable Touch ID you’d need to restart an iPhone, wait for a few days when the device automatically prompts for the passcode, awkwardly use a different finger a few times to force it to lock out, or just disable Touch ID in settings. Apple’s new method is a far more discrete way of locking out a phone, especially if you’re in a situation where you’re worried someone might force you to unlock your phone.