The principle of ESD protection devices is based on clamping the input voltage to a level that is safe for the IC to handle.
In the circuitry given using Zener diode, the ESD protection diode is “off” under normal operation. However in case of a high voltage occurring due to an ESD event, the ESD circuitry begins to conduct when the voltage is above the breakdown voltage, reducing the voltage seen by the internal circuitry.
The ESD protection device should be selected such that the device can handle the current generated due to an event, and has a resistance small enough to keep the device in reverse breakdown. Today’s ICs use sophisticated ESD protected circuitry and multiple clamping stages to ensure that the ESD rating is met.