An explanation of how differential gear works in a straightfoward and clear video. Skip to the end – does what they’re talking about make sense? Now watch from the beginning (start from 1:50 if you want to skip the intro) and see them build up from basic principles until they’ve reached the same point. Does it make sense now?
Contrast this with a written explanation:
A differential is a device, usually, but not necessarily, employing gears, which is connected to the outside world by three shafts, through which it transmits torque and rotation. The gears or other components make the three shafts rotate in such a way that a=pb+qc, where a, b, and c are the angular velocities of the three shafts, and p and q are constants. Often, but not always, p and q are equal, so a is proportional to the sum (or average) of b and c. Except in some special-purpose differentials, there are no other limitations on the rotational speeds of the shafts. Any of the shafts can be used to input rotation, and the other(s) to output it.
In automobiles and other wheeled vehicles, a differential allows the driving roadwheels to rotate at different speeds. This is necessary when the vehicle turns, making the wheel that is travelling around the outside of the turning curve roll farther and faster than the other. The engine is connected to the shaft rotating at angular velocity a. The driving wheels are connected to the other two shafts, and p and q are equal. If the engine is running at a constant speed, the rotational speed of each driving wheel can vary, but the sum (or average) of the two wheels’ speeds can not change. An increase in the speed of one wheel must be balanced by an equal decrease in the speed of the other.
(taken from Wikipedia)
While this is almost certainly an accurate description of the forces behind the device, it’s nowhere near as easily understood as a visual representation. It’s important to understand when visuals can help your explanation and if you do use them it’s extremely important to make sure it’s clear and accessible.
And lest you think these are newfangled concepts – this video is from the 1930’s!