i'll use the left round off, then reverse it for right round off.
in a left round off, the left knee in the hurdle comes up in concert with both arms. the problem that is presented is that when the left knee comes up, the left side of the body shortens and the right side of the body lengthens and takes up the borrowed weight from the left side. this lengthens the right side of the body and right arm ever so subtly.
if you watch from behind, you will see the right hip and right shoulder turn slight right. in some kids it doesn't affect them. but in others it's drastic.
so when they step down from the hurdle, and because the torso is slightly turned right (and a left round IS a right twist), when they step down on to the floor you will see their right (back kick leg) leg slightly to the left of their butt crack (center of body) and their left leg (push off leg) slightly to the right of their butt crack.
have them do it on a tape line. you will see them step down with their right foot in back on the left side of the tape line and their left front foot to the right of the tape line. this phenomena is called 'crossing over'. you will hear coaches say this all the time. they tell them to stop "crossing over" in various ways but none in which will correct the problem.
so, you want to keep the right shoulder and hip from opening to the right. you accomplish this by softening the arm swing with the right arm. you start by having them NOT swing the right arm up any higher than the chin.
from the side you will see the left arm above the forehead and the right arm slightly down around chin level. this prevents them from opening the right hip and right shoulder which causes a subtle turn to the right which will then cause them to 'crossover'. when they crossover, from behind you will see their right leg kick over their left shoulder and the left foot gets stuck (torquing and rotating the right groin and hip flexor) for a moment until gravity turns over the body which subsequently causes the body to go the right and then to the left. a hot mess.
incidentally, it is this that causes so many groin pulls. if it is really bad try this yourself. only swing up your left arm and leave your right arm at your side. and when you bend over to place your hands on the floor you still thrust your right hand to the floor to push off in concert with the left hand. this is the method i recommend and use if the kids round off is really really bad. after a couple of weeks you then reintroduce the right arm swing very softly and not allowing to go above the chin.
i dare to say that this will correct a crooked round off 100% of the time if done correctly and given the time for the gymnast to adapt.
