The flywheel is a fairly large wheel that is connected to the crankshaft. It provides the momentum to keep the crankshaft turning between piston firings. The flywheel is the base for the entire clutch attachment. The side of the flywheel that the clutch is attached to is smooth, so that it provides a surface for friction. The clutch assembly is mounted to the flywheel, sandwiching the clutch plate in between. A bearing, called the "pilot bearing" is installed in a hole in the center of the flywheel. This lubricated bearing, either a ball bearing or a bronze bushing, is used to support one end of the clutch shaft, which is also the transmission input shaft. Around the flywheel is the ring gear, which the starter motor turns when the key is turned.
Lightened Flywheel:
The actual horsepower of the engine does not change related to the weight of the flywheel. However, to increase the engine RPM, and accelerate the car, all rotating weight (as well as the entire car) must be speeded up. This requires power, and the heavier the part, the more power needed to speed it up. The less power needed to speed up the flywheel (and rotors, clutch, driveshaft, wheels, tires, car, etc.) the more power available to speed up everything else.
The trade-offs are as follows: the heavier the flywheel, the smoother the idle, the easier the starting is from a stop, and the smoother the cruising/slight throttle changes are. BUT - the slower the acceleration is. The lighter the flywheel is, the harder it is to get the car moving from a stop (just requires a somewhat higher RPM before the clutch is let out - which wears the clutch faster), depending on porting, intake, and exhaust, the idle will be somewhat rougher, and some "jerkiness" may be noticed while cruising. BUT - you will be able to accelerate faster.
The above reasons are why there are basically three types of flywheels offered. The stock units are fine for most "normal" driving, as done by the vast majority of drivers. The light steel ones are great for "spirited" street driving, middle range autocross classes, some drag racing (heavier is sometimes better), and just general high-performance applications that will still be driven on the street sometimes. The lighter aluminum flywheels are only recommended for straight racing.
