Canon EOS-1DX MARK II Settings Manual page 37

An example where [Responsive: + 1 or + 2]
is effective
When you want to quickly switch from one athlete or
subject to another, without halting AI Servo AF:
Example where a soccer player dribbling the ball passes
it to a teammate, and the player that received the ball
dribbles it in a different direction. The [Responsive:
+ 1/ + 2] setting is effective when you want to quickly
switch between players while continuing to focus with
AI Servo AF. It is effective in a variety of cases such as in
baseball when you want to quickly switch subjects
between a player running in an attempt to steal a base,
or a pitcher starting to throw the ball.
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