When you restart your iPhone, you are forced to reenter your passcode to unlock your phone. If your phone is off, this prevents anyone with access to your phone from breaking in.
But with the latest beta (iOS 11 beta 6), Apple added this shortcut:
iOS 11 is a game-changer for Touch ID. Press power button rapidly 5 times and it opens the 2nd screen, but it also forces passphrase entry! pic.twitter.com/uvWbM04lyk
— Kia☆ (@alt_kia) August 17, 2017
In a nutshell, if you press the power button 5 times quickly, you are sent to the emergency call screen (as you were in previous incarnations). But in the latest beta, Touch ID will no longer unlock your phone, forcing you to reenter the passcode to regain access.
This is a smart add. You can make this move silently, even with the phone in your pocket.