In table tennis, you can contact the ball on the rise, on the top of the bounce, or on the fall.  Sometimes beginners have difficulty controlling the spin, depth, and power because they don’t know when to hit the ball. 
 
If you are a beginner-intermediate player, I would recommend attacking from the top of the bounce, the highest point.  This will allow you to have the most power hit hitting forward on the ball.  If you attack on the rise or fall, then you need to lift somewhat.   However, when defending (with push or block), I would recommend contacting the ball on the rise.  By contacting the ball on the rise, it is easy to keep the ball low and control the ball.  This is especially important when receiving short serves.  When pushing, contact the ball early to keep it short or push quick as a surprise.  When flipping, allow the ball to reach its peak in order to get maximum speed on your flip.
 
If you are an advanced player, you should be are able to hit nearly every shot from every height.  It is vitally important to practice at multiple distances to add more variation.  However, you should have one main distance that you prefer and feel comfortable playing for each shot.