Knock Lock is a very handy and useful app (also lightweight) that does one simple thing to your device. Turns it off. The elegant way. As mentioned before, it enables the "knock to sleep" function, which turns off the display whenever you double-knock your display.
Don't worry, the "knocking point" is very small and also can be set. All you need to do is to enable the app as an administrator of the device, set the knocking point (or leave it as it is) and you're done.
No root access needed.
You can see the letter K above on the first image. That's the knocking point. In default, it's up there in the status bar. The bad thing is, wherever your knocking point is, the display will not work. Well, it will work, but it won't be functional and accessable to other apps.
Either way, it's well-designed, nice, simple and lightweight app and it's free! And it definitely worths a shot: