Movement of the tailvane is transferred to the main wing by the belt and pulley system.
Since the tailvane pulley is of a smaller diameter than the mainwing pulley, the wing moves about 15 degrees less on each tack, giving it a suitable angle of attack to drive the boat. As the wind  comes ahead, this angle of attack diminishes until when the wind is directly in front it is zero.
Before testing, I imagined that this setup would give reduced performance on windward courses, but I found in practice that the reduced angle of attack at say, 45 degrees was no handicap, and was probably exactly what was needed to improve the L/D ratio to penetrate the wind.