I don't think it's the rudder that controls the motion,  the fixing point of the line is moved from one side of the boat to the other...,(reply>), I think it is a combination of both.  Changing the side the cable is on would change which direction the craft tends to point,  the rudder would fine tune the angle to control speed by increasing or decreasing the angle of the boat relative to the flow of the river.    <br><br>Using the rudder to counter steer the craft to point straight into the flow would make it stop moving, while turning the craft to more of an angle would increase the speed.  <br><br>I think it works in a way that is simmilar to how the trim tab of a small plane works.