Canon EOS-1DX MARK II Settings Manual page 35

This setting allows obstacles that pass
in front of the subject to be ignored,
as well as focusing on new subjects
With the AF Configuration Tool on the EOS-1D X Mark
II, not only can you select from Case 1 – Case 6, but
three parameters for each can be adjusted individually.
[Tracking sensitivity] is the parameter that can be set to
change AI Servo AF's response to sudden changes in
what the active AF point (or points) are seeing.
The [Locked on: - 2/ - 1] setting tends to ignore subjects
that come into the AF point as obstacles, and continues
to focus on the original subject. Selecting - 2 results in
the targeted subject being tracked for a longer time
before focus changes to the subject now in the AF point.
The [Responsive: + 1/ + 2] setting determines that
subjects that come into the AF point are new subjects to
quickly re-focus upon. It is also effective when you want
to quickly focus on subjects that are hidden and appear
suddenly.
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