It indicates whether a turn is coordinated (slip) and the relative rate
of turn. In reality, your Triumph will probably turn a lot faster than
the aircraft.
The slip indicator is basically showing how much the driver being pushed
left or right as the car turns. A perfectly banked turn will show
zero...whether you're in an aircraft or on a banked track. The slip
indicator is nothing more than a carpenter's level; no power or anything
needed.
Looking a picture of the gauge, it looks like the turn indicator is
vacuum driven. If you apply vacuum, the needle will indicate how fast
you're turning. A modern "Standard Rate" turn means it takes two
minutes to do a complete 360-degree circle...as you can anticipate, your
car will turn a lot faster. Be neat to look at, assuming you can get
some vacuum to the gauge.
Ron Wanttaja