The Meritor Graphics team got a little lazy.....
So with nothing locked the cartoon truck would have spun the left front drive tires as soon as it rolled onto the ice.
Locking the IAD only guarantees one wheel in each differential will be committed to drive, so if a wheel on each drive is on ice they spin just like any open differential will do.
Now what the graphic does not show is that BOTH differentials need to have the DCDL to follow the neat green flow of power and traction that they show. Don't know why they left that out.
So in the case of my truck the DCDL is only on the rear drive axle. You lock the IAD first and then lock the DCDL. Using the Meritor graphic as a reference the RIGHT REAR drive tire would be forced into action. Hence the "Three way lock"