I tested the auto-tracking focusing today, at a local ski race.
Settings:
AI Servo AF Case 'A' - tracking automatically adapts to subject movement
AF priority (people) - enable
Subject switching - enable (slow)
1st image priority - focus
Initial servo AF point - Auto
Focus mode - all points active
Lens used: 100-400, with wide open aperture except where noted
Some examples --- note all photos are uncropped so you can see the relative size of the subject's head in the frame:
Focus continued to track on the back of the head:
Focus worked well when subject was darker than background:
Here's a miss. Failed in strong backlighting.
Another miss - a rare occurrence. Probably operator error!
This was more typical:
Panning shot just for fun --- not hard to get focus since it's at f/18