The Orbit Trackball uses a single red optical sensor to detect the movements of the blue sphere itself, flanked by two standard mouse buttons.
What makes the Orbit Trackball stand-out is what Kensington has stuck around the trackball itself: a scroll wheel. This is something that trackballs have struggled with for some time; while it's feasible to set up scroll wheel-style action to a dedicated button, it requires generally more finger dexterity than most users have or wish to utilise. By ringing the entire ball with a scroll wheel, it's always easily reached by hand. Or, in this case, fingertips.