Turning the screen on using the Knock Code feature
If your Lock screen unlock sequence is set to a Knock Code, tap your Knock Code on the darkened screen to automatically turn the screen on and unlock it at the same time.
When the screen is in time-out and a Knock Code is set, pressing
the Power/Lock Key 
(on the back of the
phone) displays the Lock screen (for instance, if you want to check
the time). You can tap your Knock Code to unlock the screen or turn
the screen back off by pressing the Power/Lock Key again (or double-tap
the Status Bar).
If you enter the wrong Knock Code 5 times, you'll be prompted to enter a phrase to verify that you're trying to unlock your phone. You can continue attempting the unlock sequence, or you can tap Forgot Knock Code (at the lower-right corner of the screen) to enter your Google Account log in or your backup PIN to unlock your phone.
Use a fingertip instead of your fingernail to tap the screen.
If Knock Code isn't your unlock sequence, you can use the KnockON feature by double-tapping the screen to turn it on.