differential

http://en.wikipedia.org/wiki/Differential_%28mechanical_device%29

http://en.wikipedia.org/wiki/Limited_slip_differential

Here's a mental picture for you. When I was a kid my Dad used to jack up the left rear wheel of the old model T, set the spark on the drivers steering column, then go to the front and pull out a choke wire that ran to the carb, then push in a crank handle, and with it in the bottom position, pull up, all the time hoping it would catch and fire on that stroke. If you did the push downward on the crank, and it backfired, there went your hand and or arm as it kicked back causing the handle to rotate counterclockwise. If we didn't jack up that wheel, and it did start, you better run fast because it would come right over the top of you! I got to see that happen a couple of times and seen Dad jump to the left as the old car putted by and stopped by the wall of the garage because the action of the torque on the motor starting would cause the thing to pop off the jack and then the fun began. This was using a type of rear differential called a Ruxel Rearend. Never got to pop it apart, but it was pretty much a direct drive affair, with direction directed by bands that were around certain areas of the transmission, and these were directed by one of three pedals. He learned to start the car that way because he was usually alone, and this was the safest method for the moment. Later I was schooled in the use of the pedals, and the jack stayed in the car.

If you have ever explored the world of 4 wheel drive, as they now say, SUV's, in the 40's to the 70's, we used to use directional traction tires, and in order to make the front end actually valuable for getting unstuck, we used to put the right front directional tire on with the tread pointing to the rear instead of the front. This was so that the right front would dig in and move the truck or jeep when the other 3 wheels were spinning because of the inability to grasp the soil. Discs inside the housing reacted to friction and would "sense" the speed of the slipping, and by nature of the lubricant used become more "tacky", seize the discs and transfer the torque to the axle with the highest speed, thus enabling you to move forward or backward as needed. Many of the 4 x 4 differentials, especially on the earlier vehicles, would lock up if you tried to drive off on a hard surface if there was no slippage to be had. If this happened, the best thing to do was to pop the transfer case into neutral and if you were lucky enough not to have caused a total bind between the front and rear axle systems, it would release from the lock. Otherwise, you had to jack up any wheel and soon you were on your way. Today, many of our differentials require synthetic oils, and if we put the wrong type in or mix synthetic with a standard oil we an get into trouble.

Just a couple of notes that I thought you might be interested in. The previous answers are really good.