I just tried it out. It doesn't actually work very well - only wakes the screen like 1/3 - 1/2 of the time I do it. Maybe I am not doing it right... but the proximity sensor seems to be in the top left corner of the screen.
The app reviews list instructions to set it to "tap tap tap" and "intermediate sensor sensitivity," which I have done, but still not working super well.
I wish there was some gesture I could do on the screen to wake it up (without battery drain, haha).