How zener diode maintains constant voltage across the load?

Zener diode has the property of behaving like a dc battery in ‘on’ state. If the zener diode is shunted across the load RL and the voltage across zener diode is more than the zener voltage VZ then zener diode is on ‘on’ state , and any variation in voltage across the zener diode due to variations either in supply voltage or in load resistance is not able to change the output voltage. Thus zener diode maintains voltage constant across the load.