Canon EOS-1 Settings Manual page 21

Continuous priority
OK
For illustrative purposes, all AF points are displayed superimposed. The blue assist AF points are for illustrative purposes only, and they do not actually light up in blue.
In cases where an obstruction cuts into the foreground,
and the AF tracking method is set to option 0: Main
focus point priority, focus can shift to the obstruction
resulting in a missed image. If it is set to Continuous
priority, it will continuously track the subject.
EF400mm f/2.8L IS USM 1/4000 sec. f/5.6 ISO200
21