A Flywheel is a mechanical device which is designed to store kinetic energy and smooth out the power of an engine. They are typically made of very dense metal that takes a lot of energy to get moving. But, once they have momentum, that density and weight works to keep the engine running smoothly.

Just like a flywheel, sales requires a LOT of energy to get moving initially but, once the momentum is in place, the energy required to keep it moving is diminished.