I found this good cross section of the right hand Sherman diff and final drive. Colour coding the separate rotating parts helps me understand this stuff.
The orange part is the diff carrier which is driven round by the pinion gear shown on the gearbox output shaft.
When driving straight ahead, the three yellow planet gear shafts supported in the orange carrier rotate at the same speed as the carrier obviously. The small gears at the left of the yellow shafts are interlocked with matching small gears on the three planet gear shafts driving the left hand final drive (not shown). The left hand small gears want to turn the opposite way, but they can't during straight driving, so none of the yellow shafts rotate relative to the carrier.
So the locked yellow large gears freewheel the green assembly, which includes the brake drum, at the same rpm as the orange carrier.
The locked smaller gears on the yellow shafts also drive the large pink gear at the same speed as the orange carrier, which drives the track through the pink final drive.
When the right tiller is pulled back, the band locks around the large drum and the green assembly is stopped.
This forces the yellow shafts to rotate relative to the orange carrier as they "walk" around the locked green gear.
The smaller yellow gears driving the large pink gear also "walk" around the the pink gear, which drives it at a lower speed, giving the gear reduction to the right final drive.
As mentioned above, the small yellow gears are also engaged with the equivalent yellow shafts for the left hand drive. The rotation of the small right hand yellow gears during a turn speeds up the rotation of the left hand yellow gear, which increases the speed of the left hand track.
How mere humans come up with this stuff boggles my mind.
Malcolm
