If you fly it in a stall, it will look like it is staying up, but it is falling vertically faster and moving forward slower. If you fly it faster, it moves over the ground faster and utalizes lift better. You don't dive it, just fly it faster. If it is nose heavy, the tail will be trying to lift the nose all the time causing more drag, therefor making it come down faster. If it is tail heavy, the tail is always pushing the nose down, causing more drag there too. If it is ballanced right, the tail just follows and doesn't produce lift in either direction until you tell it to to lift or lower the nose. If the plane is nose heavy and you have it trimmed for a slow speed, as you pick up speed, it will want to lift the nose more causing it to pitch up. If you dive it, after it picks up speed let the stick go. If it comes up really fast it is nose heavy, if it dives more, it is tail heavy. If it pitches up slowly, it is just about right.