The bucking bar works in-tandem with the clutch actuator bearing to transfer the signal from the clutch lever to the clutch itself. Each 2-stroke bicycle engine kit has 1 bucking bar. This part does succumb to natural wear and will eventually need to be replaced. It is installed inside the 10 tooth drive sprocket? which can be found behind the drive sprocket case cover.
If you change the bucking bar, you definitely should change the steel ball.