The starter pinion allows the starter motor to engage the engine to start the scooter. Once started, the starter pinion retracts back, disconnecting the starter motor and the engine. This stops the starter motor turning while the engine is running. Starter pinions wear out over time. A worn out pinion generally makes a crunch followed by a quiet whine noise until the start button is released, as the pinion has failed to engage the motor.
This will not fit the Aprilia Ditech.