Turning the screen on using the KnockON feature
The KnockON feature allows you to double-tap the screen to easily turn the screen on or off.
Quickly double-tap the center of the screen to turn the screen on.
Quickly double-tap the Status Bar, an empty area on the Home screen, or the Lock screen to turn the screen off.
NOTE
The KnockON feature can be turned off in the Gestures menu.
From the Home screen, tap Menu Key 
> System settings > General tab
> Gestures > remove the checkmark from the Screen on/off checkbox 
.