Shatter Defenses is a free action with every attack that lets subsequent attacks be against a flat-footed target, or does every hit, including the first on a qualified target count as flat-footed if you have the feat?
It's a free action... think of it as always on, if they are shaken they are flat footed to you.
I think it's not an action, because it kicks in even on an attack of opportunity, which can happen even if it's not your turn. IIRC you can use free actions only on your own turn.
I think it's not an action, because it kicks in even on an attack of opportunity, which can happen even if it's not your turn. IIRC you can use free actions only on your own turn.
Lehmuska is correct, Shatter Defenses requires no action of it's own, instead it modifies the result of hitting an opponent to also include making them flat footed, if they are already Shaken, Frightened, or Panicked. It's similar to feats like Selective Channeling, Quick Draw, and Nimble Moves, which each modify other actions without requiring a separate action of their own.
Shatter Defenses doesn't kick in until you successfully make an attack, so your first attack will never receive the bonus, and should you miss, neither will your second, and so on. As soon as you make a single successful attack roll (and before you roll damage) the target becomes flat footed until the end of your next turn.
It's arguable that this might allow rogues to deal sneak attack damage, even on the first attack, as the target becomes flat footed prior to rolling damage, but I'd personally rule against such an interpretation, since at the time of the attack, the target still retained their dex bonus to AC.