This is a demonstration of the Sony a9 autofocus feature. The settings were: Face recognition on, and I set the C2 custom button to activate EYE-AF. This is not the preferred way to shoot a moving ice skater, but it shows the continuous autofocus tracking, even on an eyeball.
I think it is because of the 60fps sampling rate for the autofocus calculation, and the 693 phase detection sensors covering 93% of the sensor. The percentage of coverage is very important in autofocus tracking, as subjects move all over the frame in a high speed motor situation.