Head Motion Controlled Break Out
One can think of several techniques to track the head of the user, but I found a very simple one to work fast (realtime and low cpu usage is a huge concern for a game) and reliable: simply thresholding the image for bright pixels and then averaging their position to a mean position, assumed to be the head. This position might by no means be really the head, since other bright objects might be in the view of the camera. But these are normally static, hence the only way to change the mean position is by moving the (illuminated) head. The amount the paddle is moved by a certain head movement might be different from background to background, but the human can adapt to that intuitively.
To see the technique in action, see this video: