A sprag is a wooden implement that was used by children working at coal mines (back in the days before child labor laws) to stop mine cars.
A mine car loaded with coal would travel down a slight grade to the mine entrance. The mine car had no brakes, so once outside, children would run alongside it and thrust the sprag into the spokes of the wheels. The sprag would hit the body of the mine car, thus locking up the wheel and bringing the mine car to a stop.
It was a dangerous job, often resulting in serious injuries such as broken arms or missing fingers.