When the yacht goes off course the plywood
vane through its connecting linkage rotates the servo rudder
dead ahead. The water flow forces the servo rudder to one side
pulling ropes which apply corrective helm to the tiller or wheel.
The servo rudder has only to pull the steering lines to steer.
It does not steer the yacht directly (the servo rudder does in
fact assist the main rudder in it's sideways thrust but only
by a negligible amount).
The plywood wind vane has to be feathered
'edge on' into the wind by pulling on the course adjusting lines
(call them snaffle lines). When feathered correctly the vane
is vertical or in line with the Gear. As the yacht goes off course
the wind comes round on one side of the vane deflecting it from
the vertical on its pivot shaft which in turn deflects the servo
rudder from 'dead ahead' pulling the steering lines as previously