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 engines are fitted with throttles (regulators in British parlance) 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.

In the internal combustion engine, this task is performed by cams on a camshaft driving poppet valves, but this arrangement is not commonly used with steam engines, partly because achieving variable engine timing using cams is complicated. Instead, a system of eccentrics, cranks and levers is generally used to derive the motion of a slide valve or piston valve from the motion of the main crank/big end. A variety of these systems have been devised over the years, with varying success.

A key flaw in all of these valve gears is that the same valve motion operates the exhaust valve as well as the inlet valve, making it much harder to achieve the desirable valve events on both the inlet and the exhaust at the same time. As the cutoff is reduced, the exhaust valve starts being opened for a shorter period also, reducing efficiency drastically. Thus, with traditional piston or slide valves, there is a limit to how far the cutoff can be reduced before the engine is choked by its inability to exhaust the steam.