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 (e.g., 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).
Entering the incorrect Knock Code 10 times will perform a factory reset on your device and you will lose all of your information. You will be warned on the 9th failed attempt.
Use a fingertip instead of a fingernail to tap the screen to turn the screen on.
If Knock Code isn't your unlock sequence, the Knock Code feature isn't activated, and you can use the KnockON feature by double-tapping the screen.