David Clark Isocom - how does it work?

We have this intercom unit in our 210 and it has worked fine in the
four years we have owned the plane.

http://www.davidclark.com/IntercomSys/isocom.htm

However, there is the toggle switch on the unit labeled "Isocom". We
have always left this switch in the "Off" position, because if we turn
it to the "Isocom" position no one can hear anything at all (neither
the pilot nor copilot nor any passengers nor the radios).

We suspect that the "Isocom" position is intended to isolate the
pilot/copilot from the passengers, but the manual (accessable from the
above webpage) has NOTHING on what exactly it is supposed to do.

Could someone with experience with this particular unit please answer
these questions about the "Isocom" mode, and add any other relevant
details?

With Isocom "on":
The pilot and copilot can hear each other, and ATC communication?
The pilot/copilot can hear the passengers?
The passengers can hear the pilot/copilot?
The passengers can hear the radios?
The passengers can hear each other?

--
Thanks!
John Clonts
Temple, Texas
N7NZ