(Not) Applying Feats

[darqueseid]

... What I mean is, the feat is simulating that the fighter can hit with more power by sacrificing his accuracy, ...

This (the underlined stuff) is exactly the reason why it was considered an "uncontroled" attack. The fighter just swings his weapon with extra might and hopes to hit. Less accuracy = less control of the swing.
But as i said, this was houseruled. I don't think there's a rule on this.

.. and to help the discussion, these are the penalties for TWF (from "Table 8–10: Two-Weapon Fighting Penalties" in the PHB):
Normal penalties: Primary –6, Off-hand –10
Off-hand weapon is light: Primary –4, Off-hand –8
Two-Weapon Fighting feat: Primary –4, Off-hand –4
Off-hand weapon is light and Two-Weapon Fighting feat: Primary -2, Off-hand -2

yeah I guess I didn't explain my position right,
I meant that just because you sacrifice accuracy, doesn't make it a wild swing, it may just be a slower swing.  The reduction in your base attack bonus is reflecting that its a slower+stronger swing that is just easier to avoid... 
For example: An uppercut in boxing isn't uncontrolled, its generally slower, and it does more damage.  I guess I've just never viewed power attack as a wild uncontrolled thing.
-Now the feat that lets you sacrifice AC for +damage- I'd view that as wild, because your overextending yourself etc. 

as far as the 2wp penatlies.
I was referring to the off-hand penalties when you aren't dual wielding, but those are useful too.  a 2wp fighter could swap his heavy and light weapons in order to give himself a -penalty.  Unfortunately the penalty is only to attack as far as I can tell...  you'd have to do subdual damage if you just mean to intimidate

[NunoM]

...as in many rules found throught the books, it's a matter of interpretation.

EDIT: I don't quite agree with the uppercut example. There's no question on the power of the punch, but a jab or a cross may even be slower, because you need to throw the arm farther to execute it properly.

However, the reasoning behind our interpretation was related (as i said before) to the fact that the fighter would be sacrificing accuracy for damage.

Personally, i agree with this reasoning, because the way i imagine a fighter using Pwr Attack, is a guy swinging his sword with a savage warcry and plunging the blade into the opponent as hard as he can, forgetting about any kind of finesse.
This doesn't mean the attack is slower. As i see it, it could actually be faster, because the strength behind the swing is greater.
True, this attack is not "reckless". This word is too strong for what i meant. "Less precise" is better, because the fighter is hitting harder without losing focus on the opponent.

I see your point of concentrating on the attack to build-up power and then releasing it to deal extra damage. We could even assume that the fighter loses attack bonus, because he's sacrificing speed of attack by "wasting" the time he's concentrating, meaning that an excellent fighter could foregoe even more of his speed (BAB) for a huge amount of damage. This can be interpreted like this, of course, but it sounds a bit "Kamehame" to me

Either way, the houseruling i mentioned for sparring still stands. The simple fact that the fighter is supposed to be holding back on damage to leave only the (normal + enhancement) weapon damage, negates the use of Power Attack! In fact, using or not using the feat, is (of course) a choice; but, if used, he's clearly not holding back, so all the regular attack damage applies.

 
Actually, this (kind of) answers the main topic... I couldn't find a ruling that clearly states that you can "turn off" a feat voluntarily, but it makes sense if you choose not to use it.

One could argue that there are "voluntary" and "involuntary" feats. I could even "lawyer it" up by saying: "all the feats with the "you gain..." expression are involuntary, so the character has no choice in the matter."
BUT
if the feat "needs" a "tool" to work (such as in "Wpn Specialization"), i think it makes sense that the character could choose not to use the tool properly or as best as he can.