The function of the valve is to improve efficiency when running light, with the regulator closed, such as when descending hills.
There is now a clear path through the valves and the working pistons, allowing free running for the coasting locomotive.
When the engine is running under load the valve heads are forced by steam pressure onto the support plates and work like standard pistons (Regelkolbenschieber).
If steam admission ceases or, on a steam locomotive, the regulator is closed, the pressure drops, the valve heads remain in the inside dead centre position near the centre of the cylinder due to the tension of the piston rings.
In this way, perfect pressure balance is achieved, thus ensuring excellent light-running conditions for the steam engine or locomotive.
So that the entire locomotive cylinder block does not have to be changed when the valves are damaged, they are housed in special chests.