switch mode power supply - What is the purpose of the SCR in this circuit?

I am trying to repair an old SMPS. The first step is to reconstruct the schematic and work from there on. The schematic on the low voltage side is completed for the +12V and the +5V circuits. However what is the functionality of the SCR and Zener in the circuit.

1. 
   

   Since VR4 is a zener I have been thinking of the +5 going up over the threshold of 6.2V but that would not trigger the SCR.

   
   


2. 
   

   If the +5V is going down the zener would act as a diode and the SCR would trigger if the +5V line comes below GND. But in that case the zener functions as a diode.