Canon EOS-1DX MARK II Settings Manual page 36

An example where [Locked on: - 2 or - 1]
is effective
When another rider, or a photographer, crosses in front
of the subject, focus can shift to the foreground/
background.
Example where a photographer momentarily comes in
front of the rider being tracked, then the rider appears
again. With the [Locked on: -1] setting, the camera
resists changing focus to the sudden obstacle, and
continues to focus upon the motorcycle.
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