AI mode will automatically select the pie that has the least amount of players in when arcing smash occurs, this obviously won't work if you have your entire raid clumped closely without using the entire room. This option is only suitable for 25 man and it works even if no one has the add-on but you, it may work in 10 man if everyone is standing in every section of the room.
The normal option that you see is where people select the pie where it is going to hit, this is then synch'd with players that have the addon as well in your raid (again does not require other players to work but only the players that have the addon will be able to see what you or they mark).
Make sure you have AI turned on, as it will require the raid leader to mark a section of the pie otherwise.
AI works with something like a calculation of in which part of the pie there were least players during arching smash, so it can be a little unreliable.