Engines are commonly set to increase their power output when under load. This permits low speeds but very good speed stability, so the climb performance is excellent. The original scheme used was to take advantage of the tendency of a nominally two-stroke glow engine to change from running in a four-stroke when set with extremely rich mixtures, to running in a two-stroke when the load increases. This is called a "4-2 break". More recently, tuned exhausts have been used to provide regulation of the speed in flight. Combined with propellers of relatively low pitch, this has permitted much more control over the engine's speed and power response to maneuvering. Electric system contain feedback control system that govern the RPM of the motor to a constant value regardless of load.