Both instruments are wired to the same receiver, tuned to the fixed VOR station in the plan view. Drag the airplane around the station and watch the received radial change. Then change the aircraft heading and watch what the conventional CDI does not do: it has no idea which way you are pointing, so flying the reciprocal makes it command turns the wrong way. The HSI, drawn on a compass card that turns with the aircraft, never lies.
A VOR radial is a line from the station to the aircraft, not the aircraft's heading. Drag to the east side of the station and the receiver displays the 090 radial no matter whether the nose points north, south or west. That separation between where you are and where you are pointed is the heart of VOR interpretation.
What's modelled: the full TO/FROM solution including the zone of ambiguity abeam the course, where a real receiver blanks the flag; full-scale deflection pinning at 10°; independent dot scaling for the two instruments; and a fixed station whose radial is derived from plan-view position. What isn't: wind, time, aircraft performance, DME slant range or VOR service volumes. This is still a geometry sandbox, not a flight simulator.