Press the spacebar to cycle between static and dynamic webcam detection. In static mode the webcam captures a static background image at the moment when static mode is selected. (Thus you should get out of the field of view of the webcam first) It then differences new webcam images with this stored one to determine the location of moving foreground objects. In dynamic mode the webcam is continuously updating its background image. This makes it a little easier to use and less sensitive to lighting changes or moving the webcam. However it also means if you stay still it will eventually misinterpret you as part of the background and will continue to do so for a little while after you move again.
Press either mouse button to cycle the display between reproduction of the entire webcam image and display of only the foreground objects. This last mode can be useful in seeing how the webcam sensing algorithm works and how best to interact with the scene.