http://forums.worldofwarcraft.com/threa ... 1&pageNo=1Explains the mechanic behind the new AD as it currently is on the PTR.
In a nutshell, AD will reduce any hit that takes a player below 35% or any hit a player takes while under 35% hp by 30%. Once the player is "killed" and the Guardian Spirit effect is proc'd, the player receives a debuff that lasts for 2 minutes (the CD on the GS effect). During the time that the debuff is active, no damage reduction is provided by AD. Therefore, even hits taken below 35% or those that would take you below 35% now hit for 100% of their damage.
This is an extremely interesting way for the devs to balance AD. It's still an extremely powerful ability, but now there's a "punishment" for allowing the tank to "die". At the same time, the debuff makes the cooldown timing very visible to the player.
The GS effect still does not save players against things that are intended to kill the player (ie Meltdown, falling damage, etc.).