In steam engine operation, the point at which the valve closes the steam port to prevent more steam entering the cylinder, i.e. the end of the admission cycle. Beyond this point, the natural expansion of the steam continues the push of the piston started by the admission of steam. The cut off point can be varied by the driver adjusting the position of the reverser in the cab. Cut off is referred to as a percentage of the piston stroke. It will usually vary between about 15% and 75% of the stroke. When a locomotive is starting, maximum power is required so steam will be admitted into the cylinder for as long as possible, or about 75% of the piston stroke. The engine is therefore working at 75% cut off. Once the train is moving, the cut off is adjusted in steps (sometimes called notches) until it has reached the required speed on level track. By this time, the amount of steam admitted into the cylinder for each stroke has been reduced and the reverser adjusted so that the engine may then be running at 15% cut off.