Have a read of this thread.
That references this post.
If you plug into the field control system before turning on your controller you will connect to a competition channel. If you do it the other way around (ie. turn on the controller first) then you will initially be on a pit channel and as soon as you plug in the RJ45 cable you will be switch to a competition channel. The only adverse effect this has is to cause a cortex reset so any initialization code will run again.