Can you explain a bit how the dual displacement works? Not familiar with how shifting between the two ranges takes place.
I know very little about how it's mechanically accomplished inside the motor but you can compare it to the function of a 'rapid pump' floor jack, or a log splitter. In a floor jack you often have two pumps, one a high displacement which moves the jack quickly with little strength, and one a small displacement which moves the jack slowly but with full power. At first they both work, but once the resistance on the jack causes the hydraulic pressure in the system to go past a certain point, a combination of a pressure relief valve and a check valve (together called an unloader valve, i think) allows the large pump to 'recirculate' while the small pump continues working alone.
Log splitters often function the same way.. a large displacement pump and small displacement pump work together to cause the cylinder to move quickly until it hits the log, at which point the larger pump goes into 'bypass' and the smaller pump continues to push at a much slower speed, but with great strength.
Apply the same idea to a motor (but opposite since it would be a small displacement motor to go fast and a large displacement motor to go slow) and you have a similar result.. high speed and low torque at low pressure, but once pressure reaches a certain point a valve or valves will open and exposes more of the motor to fluid pressure, which causes it to slow down but generate greater torque.
I read a little into the first PDF i found from googling it, and it looks like on a piston pump some of the cylinders' inlets are just blocked off until pressure hits a certain point, at which point fluid flows into those cylinders too, increasing the displacement of the motor, lowering rpm and increasing torque.
Dual Displacement Motor