Can you hit down on the ball?
Yes you can hit down on the ball, the key is the angle or trajectory that you hit down with. The downward path example is; think of an airplane landing if it lands to steeply the landing path it travels down can break the landing gear or allow the plan to land. If it is to steep it breaks the airplane landing gear and the plain crashes that would be the flight path of your racket as you strike the shot down to sharp. So yes you can hit down but land the plane not crash the plane.