I think we are getting there.
Dave is right, as to the term"open diff", Our front diff is 'open' the rear diff is 'limited slip'. So the center diff in the VTD is 'limited' when the clutch works. 'Open' when the clutch does not work.
The principal is the same. In our rear diff the side/sun gears are meshed with the pinion/planetary gears. the whole assembly rotates as a unit, it is only when one axle loses grip that the sun and planetary gears work to drive the other wheel. The Viscous clutch in the rear spans the two axles to prevent wheel spin.
The VTD is the same, the sun and planetary gears just rotate as a unit. The only time that they rotate against each other is when the front or rear drive looses grip, the gears will work to drive the other end.
This is when the clutch works to "limit the slip", as it just spans the two drive shafts to limit the spin.
The difference is. In the rear diff, the ratio for the sun to planet gear is the same on both sides. In the VTD the ratio of the sun to planet gears are different, to produce the torque difference.
Nomake Wan says,
< It all makes sense if the front transfer gear (front, in yellow) is connected to the input shaft (in green). This way, when the clutch (rear, in yellow) disconnects from the rear shaft (in red), torque is still fed to the front via the transfer gear (front, in yellow), but more is fed to the rear through the planetary gears' (blue) ratio.>
The drawing is a bit small, if you blow it up a bit, you can see that the (yellow) carrier that the planet gears are mounted on, the top transfer gear, and the clutch housing, are a solid unit. The top transfer gear (yellow) is free to rotate on the mainshaft (green). If it was a solid connection, we would be back to the US. Transfer system, with the front permanently driven. Not so.