This is a great educational video. I had a loose understanding of what the differential did, but this really helped hammer many points home. I now have a better understanding of the conditions why one wheel would not spin, while another would (that always seemed to evade me).
But the bigger thing that this helped me to understand was why not to drive our part-time 4WD system on dry pavement.
As I knew the front/rear axle differentials allowed the driver/passenger- side wheels turn independently, I never understood why 4WD changed it.
As a result of this video, I realized that if I look at the drive shafts as axles, engaging the 4WD system effectively locks them together, thus creating the situation a differential was designed to alleviate.
Ah-ha! A center differential is required to allow the front and rear drive shafts to spin at different speeds.