You are referring to the variable displacement piston pumps that are used in skid steers i suspect?
I didn't understand them either until i pulled a pump apart, they are so simple in design yet very effective.
This is what i know:
The main section consists if what is known as a rotating group, this is a circular piece of steel with holes machined into it with tight tolerances and have hollow metal “pistons” that run in them, the pistons have a “shoe” that swivels on the end. The whole rotating group is the part that spins around with the pistons facing in line with rotation. The piston shoes run on a yoke that when vertical the pistons do not move back and forth so there is no pressure generated. When the yoke is twisted forward or back wards the pistons generate pressure as they are moving forward in side the rotating group.
There is whats known as a swash plate the rotating group runs on, this is where the pistons force the oil on to. Forward and back wards is determined by which way the yoke is twisted, as the pistons will be forced forward at the top or the bottom of the swash plate.
Here is a picture of a rotating group fitted with pistons. The plate next to it you see is the swash plate.
I hope that is the information you were looking for, if you want anything specific just let me know, and if i know it I'll tell you.
They are very simple devices, but they do have very tight tolerances.