Zener diode function in ringing choke converter
This is self-oscillating flyback converter.

The transistor will repeatedly conduct and turn off. When it turns off the magnetic field in the core of the transformer will collapse and the output capacitor will be charged via D8. 
Simultaneously capacitor C3 will charge via D6 to a similar voltage to the output.

Feedback from L2 goes to the base of the transistor to cause the oscillation. It can only oscillate if the base voltage for each cycle goes enough positive to cause the transistor to conduct.

As mentioned earlier D6 charges C3 negatively from L3 with the flyback voltage. When the voltage is equal to the zener voltage the feedback through R3 and C2 will not be enough to cause the base to go positive and oscillation will reduce in amplitude or stop. If a load is placed the output the voltage on C3 will drop and oscillations will increase in amplitude.

It looks like L2 and L3 have the same number of turns so the voltage at the output will be approximately the same as the zener voltage.