High school game in colorado last year. I was on the sideline doing stats. On a runing play, the runner is tackled and a flag flies in on the play. The referee gives the signal for chp block and points at the defense. The coach asks what the penalty was and the referee told him the flag was for tackling below the waist.

Is there even such a thing? I've never heard of it and my question would be how are you supposed to tackle?

Appears an error was made...
 

You can tackle the runner by blocking him below the waist.

Tackling the runner can be legally accomplished by any action that is not specifically outlawed by rule; i.e. grasping of the face mask, spearing, face tackling, etc.

A chop block is not tackling below the waist. A chop block occurs when one player blocks an opponent above the waist and then a teammate blocks the same opponent at or below the knees. It sounds like maybe one defender was tacking high and the second defender hit the runner in the knees. However, that is not illegal.

Most of the rules concerning illegal blocking and illegal use of hands do not apply when tackling a runner. Except for grabbing the face mask, most anything that would be illegal to do to a player without the ball, is legal to do against a runner.

Tackling below the waist? Maybe the official meant blocking below the waist. There is no such thing as an illegal tackle below the waist except tripping which is illegal.

"Tripping" the runner is a legal act.

Really? I don't have my rule book to look it up but it was always my understanding that the intentional use of your feet to tackle (tripping) was illegal. As a matter of fact, I know I have made this call in at least 2 ball games. I haven't read my rule book in a while but since you say it is legal, I will definitely look the rule up again.

I'll save you the trouble.

From the NFHS Football Rules:
Rule 2, Section 44 TRIPPING, "Tripping is the use of the lower leg or foot to obstruct an opponent, who is not the runner, below the knee."

Rule 9, Section 3 ILLEGAL BLOCKING, Art. 7 ..."A player shall not trip an opponent who is not a runner."

"Tripping" a runner (which by definition is not tripping) is a legal means of using "legs ... by a defensive player ... to bring him [the runner] to the ground." That is in the definition of a Tackle (see 2-41).