The rule DOES say exactly that:
Hurdling is an attempt by a player to jump (hurdle) with one or both feet or
knees foremost over an opponent who is contacting the ground with no part of
his body except one or both feet.
For it to be a foul, the defender must be contacting the ground with one or both feet, and only his feet.
AB, I respect your knowledge of the game and of the NFHS rules. You might well grasp the intent of the rule better than I.
But as a matter of grammar, the definition simply does NOT require the feet on the ground. I think I can show why.
I concede that the definition is ambiguous, and adding 2 words might disambiguate. Here's the way most people here seem to be reading it:
Hurdling is an attempt by a player to jump (hurdle) with one or both feet or
knees foremost over an opponent who is contacting the ground with no part of
his body except
AT LEAST one or both feet.
And here's how I've been reading it:
Hurdling is an attempt by a player to jump (hurdle) with one or both feet or
knees foremost over an opponent who is contacting the ground with no part of
his body except
AT MOST one or both feet.
To anyone approaching the question with an open mind, the first definition is not redundant, and the second is not contradictory. That shows that the original definition is ambiguous.
This way of looking at it also suggests an easy fix for NFHS, should they wish to clarify the rule.