As mentioned above, Knockr lets you to enter any app using gestures without having to turn the screen on.
There are four types of gestures:
- Double Tap
- Swipe up/down/sideways
- Long press
- Volume up (this will quickly open the camera)
You can obviously choose which app you want to open by these gestures above.
Unfortunately (and fortunately at the same time), Knockr doesn't require root but works ONLY on OLED displays (Moto X, AMOLED..etc.). Why? It's because OLED pixels automatically turns off when they turn black. If you don't have the OLED, don't be sad, I'm pretty sure the app will continue with development and make this possible on other devices as well.
App comes as a free app available on Google Play Store.