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.
If you say 'in' her car, one would tend to think it goes in the passenger compartment or in the crankcase or tank. I'm pretty sure the consensus is that parts go 'on' a car. As in 'These fuel saving magnets are ON my car.' But 'I have Slick50 IN my car.' See the difference?