Also know as the "front end." The throttle controls the amount of steam that is allowed into the piston valves. It is a simple mechanism controlled by a reach lever on the engineer's side of the locomotive cab. By varying the amount of steam, the engineer determines how much expansion will take place, and thus, how much work (or force) the pistons will be able to do. When more steam is directed into the valves, the steam expansion is less and the piston forces are stronger—but there is greater inefficiency due to wasted steam expansion capacity. This concept, called "cut-off," means that when climbing hills or starting heavy trains, more steam is allowed to enter the pistons, but on flat terrain or at higher speeds, less steam is used.