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 on the backlight and unlock the screen at the same time.
TIP!
When the screen is in timeout 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 press the Power/Lock
Key again (or double-tap the Status Bar) to turn the backlight back
off.
NOTE
Entering the incorrect Knock Code 6 times and you'll be prompted you to enter your backup PIN to unlock your phone.
Use a fingertip instead of a fingernail to tap your Knock code on the screen.