All spins should occur on the ball of the foot. More specifically, we say that the spin happens on a three toe base: the area of the foot that stretches from the ball of the foot to the big toe and the two toes next to it.
The primary reason dancers emphasize the three toe base is that it keeps your weight over the inside of the foot. Especially when spinning, it is easy to let your weight shift to the outside edge of the foot. The outside of the foot is less stable than the inside, making spins more precarious. In fact, many problems with spins stem from letting the ankle roll to the outside of the foot. Focusing on the three toe base keeps the ankle stable and in turn improves the stability of the spin.
The Drill: Repeat the balance exercises from last week. This time, concentrate on balancing on the three toe base instead of the whole foot. You should be able to lift your heel from the ground, and there should be no weight on the small toe of the foot. To make this drill easier, think about spreading the three toes in order to create a larger platform. Also think about relaxing the knee; this serves as a shock absorber between your upper body and your feet and thus increases your stability.