(I am confused by the multiple examples in response to different people, so I'm restating them for clarification, for myself even if no one else cares or wants to comment.)
The following scenario was made in response to this comment by Artoomis: "In that case, your AoO can ONLY be with the weapon you are using. Obviously, if you did not take the off-hand penalties for TWF you do not get the benefits of TWF, and get only take an AoO with the same wepoan you used in your attack."
Scenario A: I have two weapons in hand (longsword and dagger) and make a full attack with the longsword (taking no TWF penalties).
Based on what Artoomis says, you cannot make an AoO with the dagger because the TWF penalties were not taken. He clarifies it with this, which IcyCool (post #26) and Legildur (post #27) agree: "As far as I am concerned, you either have one weapon you are using or you have two and are taking all associated penalties."
From my understanding, the following scenario is no different, which is the same one I presented to Hyp in post #52.
Scenario B: No IUS or natural weapons. Attack with longsword only (no penalties, full round action or standard action). Drop longsword (free action). Draw dagger (move action or free action with quickdraw). (To make this even closer to A, keep both weapons in hand and use an action, free or move, to 'wield' the dagger.)
IcyCool in post #71 says that I can make an AoO with the dagger and doesn't say anything about the longsword, though by his wording, I'm thinking he doesn't intend that the longsword is wielded. If it were, are there penalties?
Hyp, in post #54, says "Certainly [you can make an AoO with the dagger]. But you are now wielding a second weapon in your off-hand, so while making the AoO, you are 'fighting this way', and penalties apply."
I'm guessing IcyCool and Hyp are in agreement on Scenario B. Also, I think Legildur and Artoomis agree.
Let me restate your interpretation, though, which will (I hope) clarify my response in post #65. I'm not referring to one of the scenarios in this restatement, but please let me know if you agree.
You can be both "fighting this way" and NOT "fighting this way" in the same round. You can make a full attack without "fighting this way" (no extra attack) and then (for lack of a better phrase) 'change modes' to be "fighting this way" before your turn is over, and vice versa. No one has implied that you can change modes not on your turn (i.e. not during an AoO).
I don't agree. I think you are either "fighting this way" or not, for your whole turn that round, meaning that those penalties always apply. If you take the extra attack, then you are "fighting this way." If not, not. This is defined in the same two-sentence paragraph and I don't agree that you can logical separate the sentences to create a different definition of "fighting this way". There are not multiple valid ways to view this rule IMO.