The whirly bit is a gimmick. I did some tests on a 25cm unit and found that pretty close to the same amount of air moves through it when holding it from spinning as when it spins. If you have a close look at one, whilst the air makes it spin, it is also collecting air from outside and moving it inside the unit, the opposite of what you want to happen. I made a 3 blade fan from some HDPE and put it on the shaft, which resulted in a noticeable increase in the amount of air exiting the roof space.
Much more effective are the solar powered fans.
