How does this circuit regulate voltage?
The top left winding of the transformer is the primary power input. It is controlled by Q1 which can switch the half-wave rectified AC power across this winding. A certain amount of current limiting is provided by R5.

R5 also is used by Q2 to turn Q1 off as the current through R5 reaches a certain point. It detects the emitter voltage on Q1 via R7 and C3.

The bottom left winding of the transformer controls the output voltage of the switcher - it also turns Q1 off (via Q2) when a certain DC voltage is reached and this is governed by the zener diode.

R2 is there to allow the circuit to "start-up" and it provides bias for Q1.

The output winding runs open-loop but if the transformer is reasonably well-coupled on it's windings it will provide a half-decent regulated output.

I think R4 and C4 are to control the switching frequency of the thing but maybe someone else can offer a thought or two on this?