It could be overwhelming if we try to start with the principle on EVs, but I am thinking maybe we could try to interpret how regenerative braking works in a simple picture. By making good use of breaking and decelerating, energy is stored when the generator charge the battery, which improve the efficiency of the fuel.
In low speed, the engine is driven in all-electric mode, which is emission free as well. In high speed, all motors work harmonious and direct the engine. Also, the dashboard or some available app. can guide the drivers to charge before the battery is too empty, find the nearest charging locator, or even change to the optimal mode at the time being. Imagine how cool it is when you are driving, it is like you are talking with your smart car!