4c8765c2__School_Football___For_Dummies_High_School_Rules_to_NFL_Rules_High_School_Rule
[NFL Rule] A ball carrier is considered down when he’s touched by an opponent while on the ground. For example, if an NFL runner slips and inadvertently touches the ground, he can get up and keep moving forward. [High School Rule] 
Any ball carrier who touches the ground with any part of his body except his hands or feet is ruled down; the ball is dead at that spot.
The Rules of High School Football - For Dummies