This is an option 20g chassis weight which is designed to screw on the rear of the chassis under the center rear drive shaft in front of the rear gear box. A great tuning option. Please check out the video for more information on chassis weights.
Ran with and with out it felt a lot better with it on
These chassis weights come in handy when you want to change the weight bias of your car without moving things around. like all mugen parts the machining is high quality and mounting is easy. They also look good in the car.