While researching Monroe Nivomat shocks I found this description of how these shocks work on the AG Sachs website:
The Nivomat is installed on the rear axle in place of a conventional damper. Fully automatically and without additional electronic systems, this compact device pumps the vehicle up to its optimum ride height after only a few meters. The Nivomat takes the energy needed to do so from the relative movements of the wheel and vehicle body. For every load condition, this maintenance-free system sets the ideal vehicle height, thus ensuring a safe and comfortable drive. An electro-hydraulic auxiliary system allows the Nivomat to operate either via a manual switch or in automatic mode including when the vehicle is standing still.
This from their product brochure.