How does R5 bias the zener in this SMPS circuit?
The knee of the curve is pretty mushy on Zeners &lt; 5.1V, so this is important. 

They've used the 'Y' bin opto, so the nominal CTR is about 100%. A quick look at the far side of the optocoupler should tell you that about 130uA comes from the 2M resistance and another 240uA from the TNY chip so 370uA. That means about 370uA at the LED. 

Looking at the LED curve:



You can see that Vf is about 1.0V at 370uA. That means the current through the 680R resistor is 1.4mA, so the total current is 1.84mA. Then the voltage across the 59R resistor is 110mV. Looking at the zener curve:



At 1.84mA, the voltage will be about 3.6V. 

So, the output voltage before the inductor is 3.6 + 1.0 + 0.11V = 4.71V nominal. 

Conclusion: The resistor does not do what they said, and the nominal output voltage will be on the low side. 

Compare this circuit, which uses the same zener, no series resistor, but 150R in parallel with the LED, so the zener current will be a bit higher than 5mA.