As they rotate and flip their wrist, their rear foot is turning as they rotate and it should end up almost dragging their toes and stepping through as they do. Also have them think of also putting on a shoulder seat belt as they are following through..
Another way to get them to feel the weight shift is stand in front of them and ask them to step to throw and stop.
Now ask them to bring the hand out of the glove, ball hand down and back elbow up and circle up with the ball hand.
Then grasp the index and social finger and tell them you are going to guide them forward by pulling on the two fingers. In most situations you will feel resistance immediately. Then point out that they are not on the ball of the lead foot, the knee is not flexed (make sure the foot is angled at about 40 to 45 degrees) , the nose is not over the toes and the chest is not over the knees. Then repeat and within a few attempts they will feel no resistance in the fingers at all!
We have them start with the feet about a shoulders width apart and the glove is in what we term the gathered position or the glove is in the middle of their chest with the ball in their hand in the glove.
They are balanced and then they slide their back foot to the instep of the lead foot and step forward with the lead foot and slide step and break and throw. The break comes after they step the ball hand comes out of the glove and goes down the up and circles.
We also show them how to increase the velocity of their throwing arm by inter locking our fingers of our glove hands while we are in front of each other. I will lead and show them how to tuck me in and I will pull my glove arm back so my elbow comes in under my shoulder and rearward and they will be pulled towards me as my ball hand arm goes forward. This is a basic martial arts move to increase the force of the punch or in this case throwing arm action.
With this technique we have found throwing force speeds can increase as much as 3 to 8 MPH for most kids.