I know I used to mistake "hitting down on the ball" with hitting the ground just behind the ball. You can hit the ball alright most of the time this way but chunks and blades can happen more often too. When you hit down on the ball, the ball should be contacted first as the club travels on a steep angle and actually contacts the ground slightly in front of the ball. If you come in flat under the ball in a scooping fashion all kinds of problems can result.