This is an optional X Factory Brass Slipper Flywheel, and is intended for use with the X Factory buggy and trucks, as well as any car that uses a Associated Slipper Clutch top shaft. With the development of brushless motors, sintered rotors, and high-discharge LiPo batteries, the massive torque can be too much power for some tracks. This flywheel bolts right on the slipper shaft and rotates opposite to the motor and wheels, smoothing everything out.
On high traction surfaces where the tires have excellent grip, the added weight is not necessary and a lighter flywheel is what is needed. This is the reason why X Factory has center-punched a dozen holes along the outside of the weight and supplied the proper diameter drill bit, so you can lighten the part to suit your track conditions.
NOTE: Drilling three holes takes out about 1/2 gram from the 15-gram part.