I am a screenwriter and I wrote a part in the script that a player kneels down as the ball carrier approaches and then steps on his back and either uses him to get higher into the air or the player stands up and hurls him into the air at the same time, launching him too high for the defense to reach or stop. I have heard that this is an illegal move, which I don't doubt but I just wanted to confirm this. Does anyone have an answer to this or what they call this illegal procedure?

Actually, this was an allowed tactic in football in the first part of the last century. The ball carrier wore a thick leather belt with handles on either side. He would carry the ball to the line of scrimmage where 2 other players would grab him by the belt handles & hurl him over the line. Obviously, that procedure is now forbidden as "assisting the forward progress of the ball carrier." Another interesting fact about that time period: The ball carrier had to physically down the ball in the end zone. This meant he had to cover the ball with his body while lying prone on the ground. "Breaking the plane of the goal line" was unknown back then but so were facemasks.
The other idea of bending or squatting to allow is forbidden for the same reason. A similar tactic we used may still be in practice. My blocking guard was called "Big John" for a reason. He was so much bigger than other players. On a straight forward dive play, "Big John" two fistfuls of the othe man's jersey, lift him off the ground and then dive on top of him. This gave me 2 yards of running room up his back where his shoulder pads would be used as a launching pad into the secondary. It was good for 5 yards on every play. "Big John" was so adept at this that the officials never caught it. Everybody thought he just knocked the guy on his butt.