Here is a GREAT video about how the rear differential works. For most of us, the transmission is something we understand, but don’t know in depth. A lot of us rebuild engines, do brake jobs, tune ups, etc… but how many people rebuild their own transmission?
The rear differential allows the rear wheels to spin at different rates during turns. This allows the engine to transfer more power, and allows for more traction in slippery situations.
This is a great classic video. It’s also not super technical. Although very important, the rear differential is relatively simple in it’s basic design. There is a long intro, so you should skip to about the 1:50 minute mark.