The idea behind is that the circular bars are sharing the focal point with the reflector. It means that in every position the parabolic will have the same focus point. You dont need to change the position of the cooking pot, just push the circular bars up and down.
The two wheels on the frame serve to turn the device around. One idea is also to connect the support for the cooking pot to the frame. In this model you would also have to adjust the stand for the cooking pot as you turn the frame around.
The reflector will stay in position when the position is changed. This is due to the friction between the circular bars and the bars on the frame.
The reflector is made up a real parabol and divided into parabolical strips that is adjusted to share the same focus. This model is a conceptual design, it is not jet constructed, still it is designed and calculated accurately. This means that the parabolical sheets have a real parabolic shape and they are all sharing the same focus.