This is easily noticable when attempting "Nap of the Earth Terrain Flight" (aka TERF). While approaching a hilltop or ridgeline and attempting to maintain the same altitude while going over it, you should be able to maintain the same collective stick setting while simply changing the pitch attitude of the helicopter and "trading altitude for airspeed" and vice versa when you want to decend on the other side of the hilltop or ridgeline.