The weight should be held over the ball of the foot as you turn, however, you should not think of it as "standing on the ball of the foot", which implies that your heel is up. You can turn over a flat foot, provided there is no weight on the heel.

Imagine a piece of wood the length of your foot. If it were nailed to the floor on one end, it would be flat against the floor, but you could still pivot the wood around that end. However, as soon as you put a nail in the other end, you fix it to the floor so that it can't pivot.

When you stand normally, your weight is held somewhere in the middle of your foot. This applies pressure to both the ball and the heel, which is like having a nail in both ends of the wood. In order to pivot on your foot, you simply need to disengage the weight from one end of it or the other. But this can be done while keeping the foot flat, just like the piece of wood.

Regards,
Jonathan Atkinson
www.ballroomdancers.com