One of my favorite features in the LG G2 is the ability to wake the phone without using the power button by tapping twice on the screen. Called "Knock Knock" or "Knock On," it's sort of a necessary side effect from the power button being on the back of the phone. You might want to wake the phone to see a notification, but not necessarily pick it up to do so.
Nokia has done this in the past, and you can get the same sort of feature with a custom kernel. But this is the first time we've seen an Android manufacturer build it in.
And, so, we knock. Or tap. A double wrap on the dark display wakes the phone. From there, you can double tap again on the lock screen to put it back to sleep.