Here are the images of the centre-pull ball dyeing I did in the same indigo vat.
I already had some centre-pull balls wound tightly and only had to "wet them out". I then put a rod (long paint brush) through the centre and allowed the ball to just sit in the vat.
After about 5 minutes I flipped the ball over and let the other side sit in the vat, as you can see in the image on the left.
Here you can see the distinct areas of colour.
Below is the hank hanging on the line after washing and rinsing.