Double Tap to wake on Nokia Windows phones

I heard that double tap to wake is coming to Nokia's Windows Phones with an update. I was thinking, wouldn't it be a drain on the battery life as the touch screen is always active and "listening" for touch events? What happens currently in Windows phones when the screen is locked? Is the touch screen disabled unless you press any of the hardware buttons or it is still active and always "listening" for touch events? I am kind of a newbie to hardware/technical side so if this question sounds amateurish, then it probably is.:)