It's a terrible rule that's too widely enforced.
What kind of idiot that came up with it I'll never know.
If a guy I'd 2 feet off the ground there's no way to hit him without your head coming down.
If you are running 18mph you are covering 26 feet per second. If a offensive player has his body position changed or lowers his own head you can't recognize it register it and change your target inside of 3 yards.
It's asking the impossible and putting skirts on the ball carrier