In a steam engine, one method of obtaining the earlier cut-off with a simple slide valve may be obtained by increasing the angle of advance of the eccentric but reducing the throw of the eccentric and keeping the steam lap and exhaust lap constant. The method will cause withdrawing or throttling of steam.

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.