Although there's nothing like cycling with a good tailwind, your bike and your body are only capable of catching a limited amount of that wind. The CycleWing was designed to help, acting as – you guessed it – a sail for your bike.
As I learned in a one week windsurfing course you can get pretty speedy just with the force of wind, but you can also smack your head pretty good if the sail is not alligned propally.
I guess theoretically it could work as long as you don't have a headwind either directly on you or within the 90 degree front peripheral, but then I'm applying sailing logic which probably doesn't work on the road