It's sweet and simple: the camera captures the reflection of light on the user's eye, and image processing technology then calculates the user's viewing angle to allow for hands-free navigation on-screen.
Total Immersion specialises in so-called web mashups, where an online user sees a video reflection of themselves coupled with a 3D object that they can control, such as camera that responds accurately to button presses, or a pair of glasses that lets the user switch the frame or colour scheme.