Focus assist beam

[DeadEye]

How does it work? After a cleaning of the focus screen the auto focus on the xti acted real kooky. I blamed it on possible damage to the delicate screen and decided to replace it after the next shoot in mid August. Turns out it’s the beam emitter because if you set the menu to use only the emitter from the external flash (430ex) it works perfectly. Please educate me about the beam emitter and how it works. Thank you D.E.