Once you install and open this program, your iPhone's screen displays four colored circles of different sizes. These are the "holes" that you cover with your fingers, as you would the holes on a flute. Then you blow into the microphone hole at the bottom of the iPhone, and presto: the haunting, expressive, beautiful sound of a wind instrument comes from the iPhone speaker.
Different combinations of fingers on those four "holes" produce the different notes of the scale. (You can change the key in Preferences--no doubt a first on a cellphone.) Tilting the phone up or down controls the vibrato.
Now *that* is pretty damned cool. I haven't actually played with it myself, but I'm sure it could keep me amused for a while... assuming I could figure out where to put my fingers. :)