When the speed control pedal is stepped on and in forward, the variable swash-plate is tilted as shown in figure
above.
As the pump cylinder block rotates with the input shaft, oil is forced out of pump port A at high pressure. As
pressure oil enters motor port C, the pistons, which align with port C, are pushed against the swash-plate and slide
down the inclined surface.
Then the output shaft rotates with the motor cylinder block. This drives the machine forward and the angle of pump
swash-plate determines the output shaft speed.
As the motor cylinder block continues to rotate, oil is forced out of motor port D at low pressure and returns to the
pump port B.