It's a restriction in the air intake path, plain and simple. I've seen a "before and after" dyno run and the car lost a few HP across the board. It's dangerous too, what if one of the blades broke off and got sucked into the engine? The one I seen looked like it was made out of an old pop can.
Lastly, any swirling effect it may create will be dissapated by the time the air reaches the cylinder.