Jim is not around ...  So I will respond to his thread

Custom function 4 (page 156  for 5D (your manual? look in Custom functions section ). It controls how you initiate autofocus on the camera. Normally the camera tries to autofocus when you push the shutter release half way down (CF4.0). In this mode the * button on the back of the camera works as exposure lock.

Custom function 4.1 reverses these functions. Autofocus is initiated by pressing the * button on the back of the camera and exposure lock is obtained buy half depressing the shutter release. Custom function 4.3 is the same as 4.1 except that exposure lock is disabled.

The way I use 4.1/4.3 is to leave the camera in AI Servo all the time. As long as I have the * pressed in the camera continuously autofocuses. As soon as I release the * button the camera stops focusing. So, pressing the * briefly gives me "One Shot AF", holding the * button down gives me continuous AF tracking. If I want to focus lock / recompose then I put my AF point over the select target press the * button briefly and then release it and recompose the shot. If I then want to focus on a moving target I just hold the * button down and get instant AI Servo AF. No mucking around changing focus modes and no worrying if I have set the wrong one by mistake. Using the camera this way is the favourite trick of  wildlife and sports photographers or photo journalists. Once you get used to it, it's incredibly flexible. Like most things when you do something new it takes time to practice to get it right!! Best advice it to try and practice on readily available birds like Gulls who dont move too fast..

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt