Tto ensure the efficient working of a steam locomotive, steam has to be admitted to, and exhausted from the cylinders at the precise moment in the pistons cycle. This is achieved by valves located next to each cylinder.
Basically, the cylinder has a port at each end, and the function of the valve is to admit fresh steam under pressure at one end, while allowing used steam to be exhausted from the other. Later locomotives featured the use of piston valves as shown, in which the cylindrical valve chest contains two heads that open and close the ports in sequence.
The amount of valve movement past the port is known as lap. In slow moving locomotives, a long lap is desirable as the delayed opening of the exhaust port gives time for the steam trapped inside the cylinder to make the best use of its expanding energy against the piston.
On fast running locomotives, allowing the exhaust port to open early when the valve is in mid-position helps the steam to escape faster and so reducing back-pressure. Higher speed locomotives also utilise long lead, meaning that the admission port is already open when the piston is at the end of its movement and so supplying good steam pressure immediately it begins its movement away from the end of the cylinder.
Cut-off is the term used to denote the position of the piston in its cycle at the precise moment that the valve is closing and therefore stopping steam being admitted to the cylinder.
When the locomotive is working slowly, a long cut-off can admit steam for most of the stroke of the piston, however as speed builds up, this overtaxes the boiler and leads to back-pressure. To prevent this, cut-off can be reduced at high speeds until steam is admitted for only 15% of the piston's stroke, its expansive properties being used to push the piston for the remainder of the stroke.