Some people have gotten the right idea here. It is about terminology. It's not that the coach is really trying to get guys to hit "down on the ball". It is the fact that he is trying (I think) to get guys to be "short to the ball". I don't know that anybody is attempting to get a player to hit ground balls all of the time. If so, I disagree with the philosophy. However, it is proven and shown in video, pics, etc on the web that good hitters work in a "downhill" swing plane to get "to" the ball but the barrell must stay in the zone at a level plane. "Short to the ball, long through it" or "down to the ball, level extension through it" usually gets the point across with the swing plane that successful hitters generally have. In order to backspin the baseball (which by the way, obviously makes it travel further) a player must consistently work down hill to the baseball, contact top palm up, bottom palm down, and keep extension through the ball on that level plane. So much of hitting instruction comes down to terminology and handling each hitter in his own way. An absolute level or slight uphill swing plane as some have said would mean that a hitter would have to "guess" as to where the pitch enters the hitting zone. Do you want your hitter starting with his hands at his back hip? Doesn't make sense. So.... you have to work downhill to be short to the ball, then the plane changes through contact and extension. I see it in my own head, but it's hard to put in absolute words. A loopy or uphill swing doesn't work to stay on the ball and make it carry.