Drive belt torque opposing the rotors which want to remain stationary. Laws of physics
The pulley is driven by the belt. At the end of the pulley (input shaft) is a plate with 3 pins. On the first gear inside the gear housing are 3 opposing pins. Each set of three pins enters the coupler from each side, so the coupler has 6 holes. The plastic gives a little so you don't beat the metal parts to death when you romp the go pedal (next to the brake pedal).
It'* a flex coupler, designed to take the hammer blow.