Canon EOS-1DX MARK II Settings Manual page 18

Sometimes when the subject moves from the selected
AF point, focus can shift to the background (resulting in
an out of focus subject), similarly when an obstacle
obscures the subject, focus can shift to the obstacle. By
selecting Case 2 in situations like these, the camera will
attempt to continue to focus-track the desired subject.
When a subject moves away from the AF points for an
extended period (such as swimmers doing the butterfly
stroke, or sports where the subject is hidden for
intervals), even better performance may be achieved by
manually setting the [Tracking sensitivity] parameter to
[-2].
Case 2
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Try selecting Case 2 when shooting a track and field
event. The arms of runner closest to the camera will
interfere with the camera's view of the primary runner
(in purple), targeted with the AF point. However, with
Case 2, AF response to the sudden obstruction is
delayed, and it will be possible to continuously focus on
the intended runner.
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