Differential Steering, studebaker weasel, steering - turning 

It actually is the steering mechanism used in the Sherman M4. It is a "Cletrac" system, a true differential and regenerative steering system in that retarding one side accelerates the other. Each (magenta-colored) planet gear meshes directly with its neighboring (mustard-colored) planet gear, ensuring that they will rotate in opposite directions or not at all.

Good question, but I think it isn't. Since this 'open controlled diff' can self-steer and the triple diff can not, as I recall.

No it isn't. Its a single torsen like differential where the ratio inside can be influenced by the two brakes to steer the vehicle. Here you can see a double diff steering created with Lego technic : ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-UNdKbaA0EPQ.html

is this kind of like the one in the m4 sherman?

So this is essentially a triple diferential?

I keep looking at this, but I don't get it. What is with the little yellow "collar" gears at the base of the axle shaft - why does that loo like it's stationary, and the other gears rotate around it? I thought at first it was the braked axle, but no, you can see the axle is still rotating, so how could this gear possibly be sitting there not moving at all? Is that fixed to the yellow "brake drum" piece that is being braked? Then why is the axle shaft still turning? Isn't the point of braking one side to slow the axle shaft? And if the drum has been brought to zero rotation, shouldn't the axle shaft be also? Is that like some way of "half braking" one side, so applying brake to the drum doesn't STOP the left side, but slows it down? Because at the end, you can see the yellow collar on the left side isn't spinning, but the right side one is. Both axle shafts are spinning. I can't figure out if that makes sense or not- if you stop one side from spinning, the other side stops. If you spin one side, the other side spins in the opposite direction (like a regular differential). But if you stop just the yellow brake drum it only SLOWS one side? That doesn't seem right either somehow. I feel like there is something wrong here, and I'm just not seeing it..