T&T: A Shunt Question

[Thomas Averill]

The prior owner of our boat has devised a rather complicated system of
solenoids to tie battery banks together when oil pressure turns the solenoids
on or when a manual switch is engaged.  It has worked for three years so far
without fail.  However, it involves the use of a "Shunt" which I do not
understand.  Can you explain what this electrical circuit is doing and why it
is there?  It seems the system would work without the shunt.  It appears to be
involved in breaking both the + and - side of the busses.