do you know what a flywheel is? no its not a light rim...
take basic physics?
its all about rotational mass.
similar to sprung and unsprung weight.
more unsprung weight = more inertia.
same concept with heavy and light rims (except the flywheel is actually a hybrid weight because it does not directly force contact with the ground but it does rotate)
there was a forumula going around along time ago saying that for every lb of unsprung weight it is equivalent to something like 8 lbs of sprung weight, dont know if its fact or fiction however it is a fact that 1lb or sprung weight is not equal 1 lbs of unsprung weight.
and hondadriver... i think you were the one asking about auto your tranny... auto trannies dont have flywheels.
normally on manuals you just get the stock flywheel resurfaced, they can be "machined" down some but generally people just pick up aftermarket lightend flywheels like fidanza's etc...
no HP is gained, its just lighter. rule of thumb, for every 100lbs you shave off you should run a tenth faster at teh track