The best way to describe the rule of thirds is to picture a rectangle (your photo) split into thirds vertically and horizontally.
This rule give you for points on fours lines that will give you four focus that will emphasis the photograph. Not only does it create points of interest in create a structure for proportion.
The theory of the rule of thirds is that if you place objects on the line or between the section then the photo will become more balanced therefore will look more natural to the viewer. The eye is usually drawn to one of the sectioned outlined rather than the centre.
My Version of The Rule of Thirds
In my rule of thirds I have kept the Sannah (the girl in the film) in the first to thirds. Her head is on the top horizon line and the base of her arm so that the movement catches the audiences eyes as it moves across to the right hand third. I think this works as the focus point becomes her smoking.