Canon EOS-1DX MARK II Settings Manual page 25

In these situations, Case 4 is most effective. With the
[Accel./decel. tracking] parameter set to [+1], the AI
Servo AF will work to focus track any changes in speed,
including sudden stops and acceleration.
This makes Case 4 the most effective setting for
shooting soccer, rugby, basketball or sports where there
is a lot of running and stopping, as well as changes of
direction which was difficult for Case 1 to track. It is also
effective for cornering during motor sports (sudden
deceleration and acceleration), and starting dashes.
Case 4
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For example, the start of a track and field event. With
Case 4, the movement of the runner is captured in the
moment from the resting state to sudden acceleration.
By using Case 4 the AF system reacts to sudden changes
in speed, allowing accurate focusing of the starting dash
moment.
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