The principle is called zener breakdown.
Due to the way the diode is doped, when a reverse voltage is applied, the depletion region widens and creates an electric field



As the reverse bias increases, the electric field reaches a point when it is high enough to energize the electrons into breaking out of their covalent bonds and becoming free electrons. And these free electrons cause the high current.

The point to note here is after this breakdown point, higher reverse bias just creates more free electrons, but the depletion layer potential does not change any more. It is this potential that gets fixed as the zener voltage.

-Karthik

thanks 4 reply
but the question was y is that voltage across the depletion region constant (for voltage greater than breakdown) , is that because (i think )
when more electrons are created the resistance of the diode adjusts itself by lowering its value so there will be a compensation and hence constant voltage ,let me know is this right

In a way yes !!!

-Karthik