The way I understand it, if you are in High (rabbit) mode and do something that would bog the tractor down, it will drop you into low (turtle) mode. The other gear selection remains the same. As to how it works, you are talking to someone that wouldn't recognize a swash plate if it jumped up and bit them, but for fun I will type in part of what is in the WSM on that topic:
With loads applied on the motor shaft, the pressure between A and C is increased, and the pump receives more resistance, thereby decreasing the engine speed. Since the engine stall prevention function starts to operate first, the ECU controls the proportional control valve according to signals from the engine speed sensor and accelerator opening sensor so that engine torque and loads on the HST should keep well-balanced relation. Loads applied on the engine side are controlled by returning the swashplate on the pump side toward the neutral side. When the loads on the engine side cannot be controlled by returning the swashplate on the pump side, and the engine speed is decreased more, the Hi-Lo solenoid valve is closed by signals from the ECU to the Lo position. Then the oil between A and C (Forward and high pressure side) passes through the two speed change spool and is guided by the piston on the Lo side, and the position of the swashplate on the motor is changed to the Lo side (the inclination angle becomes larger).
There is a diagram, it looks like the swashplate is in a big chamber with pistons at 2 exits. I'd like to see one in real life some day, taken apart, just not mine...
