Inertia. The difference in mass between the stock pulley and the smaller one is not much. Once that mass is spinning, inertia helps a lot, considering it is spinning at around 800 RPM at idle.
I'd be shocked if the gains from the change in mass were more than 1/10th of 1hp.
Once a mass is spinning, as in a gyroscope or stabilizing flywheel, it takes very little energy to maintain the spin. Spin up (accelerating that mass from a stop) is where more energy is needed, but since the pulley never stops spinning at idle (inertia), the gains would be very small.
The flywheel has a lot of mass. I wonder what the supposed gains from a lightweight flywheel are? If the UDP beliefs are true (5-10hp), then the gains from a lightweight flywheel must be 5x more (since the flywheel has probably at least 5x the mass as the acc pulley).
I doubt a lightweight flywheel adds 25-50hp, lol
