Here's the way I see it, from a very basic theoretical view: the spinning mass in a 2wd car causes weight transfer; we see this most clearly when the car is in the air. There's mass spinning in the same direction as the tires, and mass spinning opposite. Essentially, you can add up the amount of weight spinning in each direction, subtract the stuff going 'backwards' from the stuff going 'forwards' and come up with a net weight transfer. (The rpm each thing spins at is very important, as is the radius of it, but I'm keeping it simple here.)
With the four gear transmission, the motor is spinning with the tires and the slipper/top shaft assembly going 'backwards'. BK and I, along with Arjan, Elvo, and a few others, decided that on some tracks there was too much weight transfer going on, that the car was pitching too violently on some surfaces. We didn't want to go all the way back to a 3-gear transmission, but we wanted to tone down the 4-gear's effect. So, we added some weight spinning the 'wrong' direction and found it felt really good lots of places.
Essentially, if you picture the 3 gear transmission on the left, an the 4 gear on the right, the brass flywheel brings the 4 gear back to left some, to somewhere right of center. If you ran the 3 gear, and added the flywheel, it would feel more like a 4-gear, but not near as much.
As to what the flywheel would do to a rear-motor car, I'm not really sure. I agree with Richard's second post, that it should increase weight transfer. It's also 15 grams of weight sitting behind the rear axle, which will add to the static weight balance of the car.
Unfortunately, the flywheels do not work well with the new V2 slipper parts. They do bolt up perfectly to the X Factory slipper hubs and the AE "V1" slipper.
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