It’s a bit like Minority report. Just raise your hand and your computer responds with an action. Motion-based controls with your hands, now more common with modern gaming consoles (PlayStation Move and XBOX Kinect), still feels a bit futuristic.
Flutter is a small tool for recognizing motion gestures on OS X, which recognizes you via a webcam (iSight or external) to control your favorite music player. After downloading it, you have work through a tutorial to get to become familiar with the hand gestures. Flutter then sits in the background, with your Mac’s iSight camera on (obviously required for running Flutter) and awaits your actions.
Related: See this video interview from Robert Scoble with John Underkoffler, chief scientist at Oblong Industries. Oblong was the company that did the actual UI for the Minority Report.