Is a ball carrier down by contact...
 

...when he lands on an opponent who is lying on the ground, but his forward progress is not stopped?

Dont blow the whistle, unless some other part of his body is touching the ground other than his hand or feet. Very hard non call to make and must be very disciplined to make this one. (or not make it)

"Down by contact" is not a HS or NCAA term. It's used in the NFL to distinguish a runner who falls to the ground on his own from one who is knocked to the ground after contact with a defender. In HS and NCAA, the cause of the runner going down is irrelevant, so there's no distinction.

The NFHS rule states (roughly) that a runner is down when any part of him touches the ground other than his hands or his feet.

No, he is not down.

Thanks all, that's what I thought.

Oh sure, just spell it out for him. :p

Here is the rule from NCAA.

RULE 4
Ball in Play, Dead Ball, Out of Bounds
SECTION 1. Ball in Play—Dead Ball
Ball Declared Dead
ARTICLE 3. A live ball becomes dead and an official shall sound his whistle
or declare it dead:
a. When it goes out of bounds other than a kick that scores a field goal after
touching the uprights or crossbar, when a ball carrier is out of bounds,
or when a ball carrier is so held that his forward progress is stopped.
When in question, the ball is dead (A.R. 4-2-1-II).
b. When any part of the ball carrier’s body, except his hand or foot, touches
the ground or when the ball carrier is tackled or otherwise falls and loses
possession of the ball as he contacts the ground with any part of his
body, except his hand or foot [Exception: The ball remains alive when
an offensive player has simulated a kick or is in position to kick the ball
held for a place kick by a teammate. The ball may be kicked, passed or
advanced by rule] (A.R. 4-1-3-I).

Why was the part I bolded added, and when? Seems superfluous.

Isn't that just emphasizing that, "the ground can't cause a fumble?"

Of course, the ground can actually cause a fumble! If a guy goes airborn and the first thing to hit the ground is the ball and it pops out, the ground did cause the fumble!

I do hate it when I hear people say the ground can't cause a fumble. I know announcers have a job to do and generally they do a good job but they don't understand the rules and spread things like this too often.

With all due respect, do you really not know what the phrase means? I think we are getting a little to legalistic. It simply means that if you hit the ground and the ball pops out, you are down because you hit the ground first. Yes, if the ball hits first, it's a fumble. So the phrase, "the ground can't cause a fumble" is accurate unless the ball hits first.

This isn't some defined phrase. It is just a phrase used by people who don't actually understand the rules.

Tell me if you think this is what NCAA meant when they added that part: That if there is doubt as to when the ballcarrier lost possession of the ball or touched the ground other than with hands or feet, the touching of the ground is deemed to have occurred first.

As LDUB said, it's a phrase that annoys educated officials because it has nothing to do with the rules. Kind of like "play to the whistle" and "he got his head in front". They are things made up by coaches or announcers that have no basis in the rules and they apply the rules using these comments.