Umi, The shaft must have a linear gear on and the prop shafts have some sort of a bevel gear on them, when the shaft is at the center or neutral position there is no pitch to the blades, when the servo is moved forward it moves the shaft and the blades move to the positive 45 degrees and in turn when reversed the props go to the negative 45 degree, the blades are treaded and must be screwed into the hub. hope that make sence.
Tug