When a Tetori uses stunning pin, does he have to declare he's going to use stunning fist before the pin or immediately after the pin? Stunning Fist can be a swift action for the Tetori if his opponent is pinned and has Stunning pin. How would that attempt work? Do you roll like a normal unarmed strike melee attack? Or are you rolling CMB vs CMD and choosing the damage option? Lastly once the damage has been done it is my understanding the enemy makes a Fort save or is stunned.
Benefit: Whenever you pin an opponent, you can spend a swift action to make a Stunning Fist attempt against that opponent.
Turn 1: Tito the Tetori grapples an opponent.
Turn 2: Tito the Tetori maintains the grapple and pins the opponent (both as a standard action), then as a swift action attempts a Stunning Fist. Tito attacks with an unarmed strike; if the blow connects (which it probably will, given that the target is pinned), the target is stunned unless it succeeds at its Fortitude saving throw.
You don't declare that you're using Stunning Fist (by way of Stunning Pin) before you pin the target--indeed, you couldn't even if you wanted to, as it triggers off of pinning an opponent. You do declare it before making the unarmed strike's attack roll; if you miss, the attempt is wasted, though you still have a pinned target.