Flywheels store rotational kinetic energy. They take some time to spin up to the input speed, but when they do, they keep spinning for some time, providing a more steady power output to a machine tha a shaft hooked up to a varying input. Heavier materials make better, more efficient - and more powerful - flywheels. Spinning one of these too rapidly will result in a violent and destructive failure.
(Sample setup needed)
The Flywheel requires a Mount under a Wood, Stone, Iron or Gold Flywheel Core