The Defensive Roll feat states:
You receive a bonus to your Toughness saving throws equal to your rank, but lose your Defensive Roll bonus whenever you are denied your dodge bonus or unable to take a free action unless you are able to succeed on a Concentration check (DC based on circumstances, see Concentration in Chapter 3).
1) Is the “unless you are able to succeed on a Concentration check” part meant to apply to both of the antecedent circumstances?
A) Yes, you can succeed on a Concentration check to keep your Dodge bonus either when denied your Dodge Bonus to Defense and/or when you can't take free actions. Then presumably:
B) A character who is attacked by an opponent after a successful feint can keep use of his Defensive Roll feat by rolling a Concentration check?
C) A pinned character (since a pinned character doesn’t get his Dodge Bonus) can keep use of his Defensive Roll feat by rolling a Concentration check.
D) A character who is Paralyzed and helpless can keep use of his Defensive Roll feat.
Are these correct? Problem: C and D in particular don’t really make sense as “yes, you can do all this”.
E) No, it’s meant that you lose your Defensive Roll bonus automatically when denied your Dodge Bonus to Defense but if you can’t take a free action and still retain your Dodge Bonus to Defense (e.g., you’re Dazed) you can roll a Concentration check to keep your Defensive Roll bonus.
Problem: if E) is right, this means that the first clause will automatically deny you your Defensive Roll bonus whenever you are stunned (since stunned characters lose their Dodge Bonus to Defense), which seems like the exact circumstance in which you should be able to keep your Defensive Roll bonus with a Concentration check.
Personally, I think Defensive Roll deserves errata here and that it should work as in E but with an exception that lets you make a Concentration check to keep your Defensive Roll feat active if you lose your Dodge Bonus due to being stunned.
2) What is meant by “DC based on circumstances, see Chapter 3”?
It is very unclear if there are any “Circumstances” in chapter 3 that would give a different DC for this check based on different circumstances.
3) In particular, I had always been playing this as a Concentration check with DC= 10+Defensive Roll rank to keep the feat active when you are stunned or cannot take actions, much like how a Force Field would work. Is this the correct way to do it? If not, what is?
4) I have also assumed that things like “vigorous motion, entangled, etc.” listed in the Concentration skill descrption on page 44 don’t independently affect Defensive Roll unless those circumstances result in one of the two listed conditions (no Dodge Bonus, or unable to take free actions). Is this correct?