The idea is that the human brain finds it hard to spot small changes to a face if that face is the wrong way up. You can mess around with a photo of someone, but while that image is still displayed upside-down, the changes won’t be obvious. Only when you flip the face back the right way again do the changes leap out and make people go “Euuuw!”
That’s what Evil Twin is all about. Take a photo, tell the app where someone’s eyes and mouth are and boom – ugliness happens.
It’s not a pretty app, but then the images it creates aren’t that pretty either. It’s a joke app, a dollar’s worth of fun to share with your friends on a drunken night out. Hic.