Click to ROOT even without PC? How is this possible?

There are many apps which says you can root in single click, like framaroot, one click root, kingo root. I'm not sure whether everythings works, but, YouTube videos claim that. How these apps root a phone with just a single click? If a app can root a phone with a click, it can also do that without the knowledge of a user. So, How do these apps work?