Before I come up with my opinion, and what I do, there is just one topic we need to get around, namely the center of gravity, which of course has an impact on the balance, and therefore how we move. Balance and center of gravity are essential when turning.
In short, you have to move your center of gravity to maintain balance.
You might not think about it; when you are walking, your center of gravity is changing every time you take a step forward. You also have to change the center of gravity when you crawl out of bed in the morning, or when you get up from a chair.
The example with the chair is good. When you get up from the chair, you will move your upper body forward and possibly move your feet backward. Without doing that, it is not possible to get up from the chair.
(If you can, I would like to see a movie about it! It would be fun).
Back to the question! What I do depends on the situation; therefore, I use the heel, the ball, or the center of the foot while turning. That way I can turn best while maintaining my good balance.
What I did! I chose to film myself while performing Kata. Subsequently, I reviewed my recordings. I saw them at normal speed, slow motion, and frame by frame. And guess what! I use every part of the foot, be it the heel, ball, or the center of the foot for turning.
Sometimes I start by turning on the heel, to subsequently finishing the turn on the ball of the foot. To me, it feels natural, and therefore it should be this way for me.
To keep the balance, I have to use the whole foot, since my center of gravity is shifting, whether I need to turn on the heel, the ball, or the center of the foot does not matter.
This is an example from Pinan Sandan starting from Nukite Chudan, following up with the backward turn.
Picture 1, 2 and 3, turning on the heel.
In picture 4, the whole foot is on the ground, and the ball of the foot takes over in pictures 5 and 6.
I would like to know how YOU turn in connection with Kata, do you use the heel, ball, or the center of the foot. Maybe you can post a video or some images.
After all, there is no wrong or right way to turn, or is it?
Take a look at Hirokazu Kanazawa turn