How are potentials developed between secondary windings in this circuit?
You're just misreading the schematic.  The winding isn't isolated - the blue (5) wire is connected directly to the + Red output (along with the sense line for that output, white-red) when the standby switch is closed.  When it is open, the circuit is in standby.  

Pin 3 of the voltage control potentiometer is connected to the "ground" of the isolated winding, and since the blue winding is connected to the red output, which is not galvanically isolated from the transistor (though it does connect through a number of components), there you have it.  You need two connections for current to flow between two isolated windings, and you have them.  They are no longer isolated.  

The dotted lines with "J" just mean jumpers, and are there to indicate some sort of wire jumper is used, but it's otherwise the same as a solid line connecting those two things. I suspect that's what's causing you problems.  If you ignore the jumper connections, then it does indeed appear to be an impossible circuit (and is - it won't work!).