How does "imprinting a voltage" work?
Remember that a capacitor acts as a open circuit for DC, and as a short circuit for (high-enough frequency) AC. Complementarily, an inductor acts as a short circuit for DC and an open circuit for (high enough frequency) AC.

So you can construct a bias tee like this:







simulate this circuit &ndash; Schematic created using CircuitLab

Now the load sees a DC current coming from VDC through the inductor, and added to that an AC current coming from VAC through the capacitor. 

The circuit you described in your question works essentially the same way, but possibly a bit more elaborate (but since you haven't shared the circuit I can't say exactly how).