The following is a diagram of a cross drive steering transmission component. The labels are in German, but my following explanations will be in English.
The dark green shaft (1) is powered by the engine. It is linked via the center shaft (2) to the two sun gears of the planetary gears. The carrier of both sun gears goes to the respective output (3) (sprocket for a tank chain, ...).
The interesting part is now that ring gears (4) can be adjusted. The light blue shaft (5) is driven (or not) by a hydraulic motor causing the ring gears (4) of both planetary gears to move in opposite directions. This allows both outputs (3) to run at different speeds depending on how fast the hydraulic motor drives the blue shaft (5). When the hydraulic motor is completely idle both tracks will have the same speed and the vehicle drives forward.
The hydraulic pump providing pressure to the hydraulic motor is operated using two levers controlling two valves that open a respective hydraulic cycle to the hydraulic motor rotating it into one or another direction. When both levers are disengaged a third cycle exists just to freely pass the oil back to the pump without going to the motor at all.
This is now leading to my question. When the tank is driving forward no pressure is applied to the hydraulic motor at all so the ring gears (4) are not getting rotated at all (and can move freely, albeit into different directions). Since both the ring gears (4) and carrier (3) are not stationary ultimately the driven gear (ring or carrier) will be the one with with less resistance.
The maximum resistance of the carrier (3) will be the force required to start moving the vehicle, the resistance of the ring gear (4) comes from the coupling between the two purple (6) axis preventing the ring gears from turning the same direction.
Is it correct that it is simply the coupling between the hydraulic motor and the two purple axis (6) preventing the ring gears (4) from moving in the same direction that allows for forward movement of a vehicle? It seems to me that this causes a lot of force to be applied to this coupling (6). Is there something else that I am missing?