http://en.wikipedia.org/wiki/Differential_(mechanical_device) this should give a good idea of whats going on
And you can still turn without a differential. It's just a bit harder on the tires :16suspect when you do so with a spool in place of the differential.
One of the biggest problems is that, if you ever loose traction with your front tires (like getting them in gravel), when making a 90 degree turn, that always locked solid rear will keep pushing your car straight even with the front tires turned all the way to the left or the right. I found this out when I had a Jeep CJ with a posi rear that must have had something seized in the unit. Even with the slightest bit of snow, not letting the front tires traction to the left or right, would cause the rear end to push the Jeep straight off the road with the front wheels cranked.
But that's now called "Drifting" A spooled rear can also be "interesting" :16suspect when downshifting to a "too low" gear at too high a speed.
Wow, things have certainly changed. I always called that "looking dumb to anyone watching your car as it's "drifting" straight into the woods out of control"