Milo,
What did you think of the video?
Cool eh!
First time I saw it was my first understanding of how a diff actual works.
So now you see why I say the other hidden wheel in your video must be turning as the one showing was slowing down and the rears wheels stayed a constant speed. The diff was trying to spin which ever wheel was the easiest to turn and at times that would have been the right front.
So just like in a left hand turn, the left wheel turns allot slower the the right outside wheel and the diff its doing it's job.
I had a 4wd Bronco years ago and thought it had 4 wheel drive. Naive as I was. Later learning that one wheel drives in the rear and 1 wheel drives in the front when things get slippy. Each axle only turns the wheel that is easiest to move and that damm it is always the one on ice or with no traction.
My preference would be to have locking devices for the front and back axles so you could switch to any amount of drive wheels.