Canon EOS 5D Mark III Instruction Manual (en)

 Selecting AI Servo AF Characteristics (For a Subject)N
Accel./decel. tracking
This sets the tracking sensitivity for 
moving subjects whose speed can 
suddenly change dramatically by starting 
or stopping suddenly, etc.
Suited for subjects that move at a fixed speed.
[+2 / +1]
Effective for subjects having sudden movements, sudden 
acceleration/deceleration, or sudden stops. Even if the moving 
subject’s speed suddenly changes dramatically, the camera 
continues to focus the target subject. For example, for an 
approaching subject, the camera becomes less prone to focus 
behind it, which would result in a blurred subject. For a subject 
stopping suddenly, the camera becomes less prone to focus in front 
of it. Setting +2 can track dramatic changes in the moving subject’s 
speed better than with +1.
However, since the camera will be sensitive to even slight 
movements of the subject, the focusing may be unstable