On the ground, at low rpm the prop is not in the constant speed 'range' and it stays fully fine. Rpm is controlled by the throttle and it functions as a fixed pitch prop. As you increase the rpm the prop enters the constant speed range whereupon the blue rpm lever will start to control prop pitch and therefore rpm. When you do the runup checks on the ground, you check the operation of the prop pitch mechanism - on the Arrow at 2000rpm I think. If you try it at say 1200 rpm the prop lever will have no effect, likewise in the descent at low power settings. I remember being very confused by it all at the start but it does fall into place, don't worry. Think of it as the gear lever.