Well before I tell you how to AFK I want you to understand there are 2 ways you can be kicked out of BGs 1 not moving for 5 minutes ,and the 2nd people reporting you. This exploit only helps you from first, so you will most likely be kicked out of a BG by being reported if you AFK with this exploit at a graveyard.
WARNING: If you have a debuff with a cow icon you have been reported AFK, do not complain if you get kicked with that debuff. A player reported you AFK.This exploit only helps you not be kicked from a BG for going AFK for 5 minutes
Step 1: Go to your options and turn off "auto clear AFK"
Step 2: Type /afk or go AFK.
Step 3: Q for a BG and Go into the BG, you will be AFK (If you wanna make sure you are AFK talk in gchat or battleground chat and it will have "away" next to your name but if you look at your toon it will not have AFK above your toon but you are AFK)
Step 4: find a spot where you will not be reported and AFK. You will not be kicked for being AFK longer than 5 minutes
I'm guessing this is more of a bug than exploit because you are AFK in the battleground so you cannot go afk if you are already AFK and will not be kicked from the BG, it will be fixed soon most likely but enjoy while it last.
SIMPLE EXPLANATION: turn off "Auto clear AFK", Go AFK, Q for a BG, Go in the BG and AFK. Make sure you don't get reported AFK.