There are a lot of spinning parts in an engine, but one of the crucial items for helping to produce electricity is the flywheel. In some engines it's simply a spinning drum; in others it has fins to also help produce air flow. However, the main function of a flywheel from it's earliest design was not for electricity but as a mechanical balance for the crankshaft.
A flywheel essentially looks like a wheel. The design generates energy equivalent to the amount of inertia or spin it has on its own. This energy in an engine can be used two ways: It can offset pressures against the crankshaft from the pistons and transmission connections, and it can be used as a generator due to its spinning feature. Many early engines ran purely on flywheels to generate their electricity, which is why their lights dim and flicker when the engines go into idle speed.
Flywheels are usually directly attached to an engine crankshaft. This is done via a tight fit held in position by a metal part or tab and then bolted down with a nut. When loosened, the flywheel usually can't be removed by hand. A puller of some sort has to be used to leverage the flywheel off its connection. The flywheel itself is a significantly heavy wheel designed to rotate as the crankshaft it is connected to spins. The rotation creates energy when magnets and coils are installed inside the flywheel to function as it spins.
Depending on how fast it spins and how large and heavy the flywheel is, a certain amount of energy can be produced. Mechanical engineers found this to be an added bonus from the flywheel, in addition to being a counterweight on the stress applied to crankshaft. As a result, many early engines ran wiring directly from the energy production of the flywheel.
Later designs used batteries to provide full power, even when the engine was idling or off. However, the application still works by recharging those same batteries when the engine is revving at full speed. Thus, automotive batteries can easily last up to five years or more with such a system or recharge.
Given the small size of motorcycle engines, they have to produce quite a bit of power in small application. This creates a lot of heat. Many early motorcycles, such as those from BMW, depended on air passing through the engine while driving to cool the engine. However, this approach didn't work well when sitting at idle or low speeds. The problem was solved by adding fins to the engine flywheel and placing enclosures over the combustion chambers. The forced air from the flywheel would be pushed toward the chamber, with increasing pressure as the engine revved higher. The cooling effect provided the needed temperature protection.
