MAKE’s photo editor, Sam Murphy, sent us a link to this 1930s Chevrolet educational video which describes how a differential gear works. Skip ahead to 1:50, if you don’t want to see the rest of the video (which is highly entertaining throughout).
How Differential Gear works [Thanks, Sam!]
Low Center Drive = Hypoid Gearset which is more of a sliding contact requiring EP sulfide additives to give the heavy oil greater shear strength.
Pretty good visuals, now we need to add limited-slip and positraction!