This pump has several pistons in cylinders arranged parallel to each other and rotating around a central shaft.
A rotary valve at the opposite end from the swashplate alternately connects each cylinder to the fluid supply and delivery lines.
By changing the angle of the swashplate, the stroke of the pistons can be varied continuously.
If it is at a sharp angle, a large volume of fluid will be pumped.
The displacement is not positively controlled, but decreases as back-pressure increases.