To my knowledge, they reduce pressure to a set psi. I've never heard of one taking a set amount of pressure head off.
In the most basic ones, a small diameter lateral line acts on a diaphram. The diaphram has a spring providing counter force. The operator sets the range of motion of this diaphram by adjusting the spring tension. The diaphram acts on a valve stem downstream. The valve opens or closes as needed, and provides increased or decreased pressure drop, as needed.
Engineering is the practice of the art of science - Steve