These devices are used to push any cars that stop before reaching the string of cars that already stands on the receiving tracks. In fact, when they are at rest they are positioned at the very beginning of the receiving track. When we still had hump yards here in Italy, before some genius decided that unit trains were the future, they were called "spintori" (pushers).
Even though modern hump yards are equipped with a series of sensors and devices that should predict where the car would stop, some unpredictable factors still remain (e.g. a gust of wind or simply a car with greater friction than expected, perhaps due to the condition of its bearings)
