Most of what you need to know can be seen on this site. Go to Cha Cha then New York. Now watch the action of the legs and feet. I think it would be preferred if you think of ball flat instead of toe heel , so we have BF. T. BF. Look at the demo. mentioned. You will see that as your feet close, you have on the closing foot a high instep. With the feet together and a high instep your knee will be in front of your body. Also look at the practise suggested on the Cuban Motion for the Rumba. All you need to do is push of the high instep and nail it. The long straight leg side , the hip should finish higher than the supporting leg if you keep your toe pointing to your side. In our studio we have almost as a ritual, moving sideways, step BH then the high instep on a the closing foot, then the push out . Keep repeating this, then try the other side.Then if you can get a more exagerrated action by turning your hip slightly and back again . I find this terribly hard to cover all the points needed. I wouldn't like to write a technique book. Hope this helps.