I'm trying to find the magazine that had the article in it, i've got it in here somewhere.
I'm no stranger to modifying 2 stroke dirt bikes myself but i'm no physicist so i'll state it as it sort of makes sense to me, i realize alot of the time tuners will remove rotating mass because it makes the motor rev quicker which is a desirable trait especially in a quad but on a 2t MX bike which is not very tractable at the best of times you can get good results adding weight to the flywheel. I've got a CR which i've added flywheel weight too and it's much easier to ride now, might not rev as quick but gets more traction and puts it's power down smoother.
As far as the claimed HP increase, i haven't dyno'd my bike but the article i read claimed an increase and i would think the extra energy generated by the extra weight spinning at full throttle translates directly to HP.
I'm gonna have to find the article though and post it up.