From Art's link above to Canon Professional Services

C.Fn III -2 – AI Servo first/second image priority

For AI Servo shooting, you can adjust the priority for the first shot or the second and subsequent shots. It deals with two issues:

1. How quickly the camera will fire if you suddenly press the shutter button fully.
2. If the shutter button is held down for continuous shooting, whether AI Servo AF will always take time to ensure correct focus for each shot (which may result in slowing down the drive speed), or whether the camera will always fire at top fps speed (even if proper focus cannot be ensured for each shot in the sequence).

There are four settings:

* 0: AF priority/Tracking priority – this will give more time to achieve focus before releasing the shutter and then during continuous shooting, focus tracking will be given time to keep accurate focus.
* 1: AF Priority/Drive speed priority – like setting 0, the first shot will prioritise focusing. However, after that the camera will try to maintain the maximum shooting speed. This is useful if the subject is large and easy to focus on, or not moving over great distances.
* 2: Release/Drive speed priority – this will fire the shutter as quickly as possible for the first shot, not giving as much time to finding focus. For subsequent shots, the camera will continue to fire at the maximum frame rate in preference to giving time to focus driving. If you have to capture the moment at all costs, this is a good option to use.
* 3: Release/Tracking priority – this will work like option 2, in that priority is given to releasing the shutter, but for second and subsequent images, the camera will then pass priority to focus tracking, rather than trying to maintain the maximum frame rate.