Jay:

The little indicator really doesn't go "up and down". The painted line
(which is probably faded by now) is a line drawn on the "corkscrew"
of the rudder trim knob. As you turn the knob, the bolt (or post or
whatever it's attached to -- check the parts manual for the
exploded diagram) is rotating, and the line moves. If I remember
correctly, as you turn the knob clockwise, the line "moves up" in
the little window.

have fun!