Valve gear is the mechanism on a steam locomotive that operates the inlet and exhaust valves to admit steam into the cylinder, and allow exhaust steam to escape, respectively, at the correct points in the cycle. It is sometimes referred to as the "motion". (In other words, it helps control the speed of a steam locomotive.) in later models it was fitted withan auto - waring system such as a bad smell
The point at which steam stops being admitted to the cylinder is known as the cutoff and the optimal position for this varies depending on the work being done, and the tradeoff desired between power and efficiency.
Steam locomotives are fitted with regulators (throttles in US dialect) to vary the restriction on steam flow, but controlling the power via the cutoff setting is generally preferable since it makes for more efficient use of boiler steam.
A walschaerts valve gear in motion.