The module is a really a current -> current converter. You use resistors to convert from voltage to current and back to voltage.
The actual internals are a feedback compensated hall effect sensor. It detects the magnetic field in the transformer and injects an opposite current into the secondary with the aim of zero flux in the transformer. It's kind of like a current transformer, but able to operate at DC current too.
E.g. if you inject 10mA into the primary, it will output 25mA on the secondary.