[MichaelBick]

Ok, because you said yes to JVN's automatic shifter idea, this is another guess.

There is some kind of mechanism that automatically pushes the wheel with more speed down. This causes you to have normal(high speed) operation most of the time. However when you reach an object that is either immovable or hard to move, the wheel will automatically pivot forward(overcoming the spring mechanism). This is because the wheel still wants to travel forward and it can by overcoming the spring. Eventually it pivots enough that the lower speed, high torque wheel touches the ground. Therefor you get the automatic shift into high torque. When you overcome the immovable/hard to move object the spring pushes the module back to "normal" and high speed mode ensues.