When the load on the engine increases, it becomes necessary to increase the supply of working fluid and when the load decreases, less working fluid is required. The supply of the working fluid to the engine is controlled by a

The valve gear of a steam engine is a mechanism that controls the inlet and exhaust valves. It also serves as a reversing gear. 

The reversing gear controls the direction of the engine's travel, and adjusts the timing of the slide valves. It also controls when and how long the intake- and exhaust-ports of the cylinders are open. 
A slide valve engine has a constant direction of pressure. It's only reversed by changing the valve position relative to the crank shaft.