Flywheel energy storage (FES) works by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational energy. When energy is extracted from the system, the flywheel's rotational speed is reduced as a consequence of the principle of conservation of energy; adding energy to the system correspondingly results in an increase in the speed of the flywheel.
NASA G2 flywheel
One of the older gyrobuses parked in a museum in Antwerp.
A Flybrid Systems Kinetic Energy Recovery System built for use in Formula One
A flywheel is a mechanical device which uses the conservation of angular momentum to store rotational energy; a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, assuming the flywheel's moment of inertia is constant then the stored (rotational) energy is directly associated with the square of its rotational speed.
Trevithick's 1802 steam locomotive, which used a flywheel to evenly distribute the power of its single cylinder
A mass-produced flywheel
A Landini tractor with exposed flywheel