shaft 8 as shown in fig1 the fixed unit block and piston set 18 begins to rotate relative to the variable unit block and piston set 12 because fluid starts to flow between them in this instance the fixed unit block and piston set 18 acts as a pump and the variable unit block and piston set 12 acts as a motor the direction of rotation of the fixed unit block and piston set 18 is opposite that of the variable unit block and piston set 12 so that the rotational speed of the output shaft 10 is that of the variable unit block and piston set 12 minus that of the fixed unit block and piston set 18 likewise if the control system senses that the rotational speed of the rotational speeds of the variable speed power source and the input shaft 8 deviate below the set point the control system adjusts the position of the variable unit wobbler 16 at an angle from normal to the centreline of the input shaft 8 opposite to that shown in fig1 the fixed unit block and piston set 18 once again begins to rotate relative to the variable unit block and piston set 12 because fluid starts to flow between them in this instance the variable unit block and piston set 12 acts as a pump and the fixed unit block and piston set 18 acts as a motor the direction of rotation of the fixed unit block and piston set 18 is the same as that of the variable unit block and piston set 12 so that the rotational speed of the output shaft 10 is that of the variable unit block.