In the French line 6. bxc3 Ne7 7. Qg4 O-O 8. Bd3 why 8. ... c4 is not played?

It completely safeguards the g pawn. The point is that white has no attack, usually in the french, that means black is better.

I LOVE this Aquarius guy, he is so funny! 

Its like the Backyard Professor, when you combine unknowingness with an oversized ego XD.

Hey, petrosian played it, and he was world champion!

Your statements are ridiculous to put it mildly .

I also play this latter idea. Not quite this but similar. The idea is that you play 4. b6 ?! with the intent to later play Ba6 to exchange the bad bishop and you are prepared to retreat the ds bishop on either a3 or Qg4. Qd7 is also a useful a move in this variation. This is a very double edged weapon, but has a logic and has the element of surprise.

Black looks for a position something like this:

i like the b6 Ba6 idea. I dont get Qd7 here though. I mean normally you play Qd7 when otherwise white had Bxa6 Nxa6 Qa4 check winning the knight. 

Qd7 is a waiting move to do something useful while white wastes a tempo for developing the ls bishop. If you play Ba6 outright then white simply takes and develops with lightning speed. Black cannot afford giving white an extra tempo in this line which is shaky anyway.

I would like to talk about what Aquarius550 posted.

Strangely enough I have looked over this line in some detail. Simply because I liked the idea Petrosian had.

I have gone through this line some more. However, those are the moves I remember off the top of my head. It seems like white has managed to find a way around the obstacles and come out with a nice little advantage. I can't remember how much the advantage was been a while since I checked it on an engine. However, I think it was half pawn or something like that.

watcha this doesnt explain anything, because Qd7 is in no way a usefull waiting move . And white can meanwhile develop sensefully. X Players explanation makes some sense although it looks really strange :D. However i hate the french so Im sure you know what you´re playing.

 Oh I see. I didn't realize they were the same line. Ya, I showed Bf8 cause that's all Hiarcs plays (>.< Computers >.<). But ya, there is a famous game from Tal - Botvinnik 1960 game 1.  I don't think the line was played much past that. For some reason the line Unzicker played versus Estrogen(?) supplanted it. Note that in the Petrosian Variation, the queen eventually stops guarding the g pawn when the knight is developed.

@ I-AM-YOUR-GRANDPA

Ok, Qd7 is not objectively the strongest move, but the engine wants to play h6 instead, which is contrary to the whole idea. The engine does not understand what your play is all about. This is an objectively inferior line as a whole, true. This is risky, true. Once you play this you have to stick with the logic of this weird system, no matter what the engine says.

c4 is an idea in this line. I just think that people want to keep their options open.

After c4!? Be2 Qa5 Bd2 Qa4 we reach a rather standard position in this variation. c4 is OK.

Qd7 prepares f5, as XPLAYERJX said.

Exactly.

I want to make clear when the Petrosian Variation starts so people are not confused.

The move 4...Qd7 is the starting position of the Petrosian Variation. The reason I am saying this is because white actually has other options at move 5.

I showed an example of 5.a3

However, 5.Bd2 has been tryed in that position with few other move's.

I honestly don't know what move is the very best.

I believe both have been tryed by higher level players. I always believed the move 5.a3 in my mind was slightly better compared to 5.Bd2.

The main reason I believed this was because 5.a3 was a little more forcing than 5.Bd2.

I always thought 5.a3 put a little more pressure on black making them come up with a decision immediately. --Either take the knight or retreat the bishop--

I don't think anything is wrong with 5.Bd2. It seems like a perfectly reasonable move. It is more of a quiet move just unpinning the knight. Making it so black can't damage the structure.

Of course not, but the jokes on you and everyone else as it seems Bc8 is not a petrosian idea but a Rybka idea. ;) 

I had no idea myself.

EDIT: I like your Nb8 idea, but I wonder if the knight would be better on d2 or e2...hmm

My only ever win against a titled player ( FM ) was in this trash line I mentioned earlier ( with the moves b6, retreat the ds bishop to f8, Qd7, Ba6 ). He thought I was a complete idiot ( which by the way in general is not far from the truth ) and started pushing the a pawn trying to punish me for playing b6. He finally managed to trap his queen and resigned.

Here is the game:

I played several games with this FM and in the other games when I tried the conventional defenses I was beaten to death.

Opening matters. I'm quite convinced that this win was due this unconventional opening choice. ( Of course I could not have won a second game in this line against the same opponent, where the element of surprise would have been no longer there, so this does not prove anything about the objective merits of this opening. )

Opening is not everything, but looking at the amount of preparation super GMs put into openings it is quite an important factor. May be a Magnus Carlsen with all his natural talent can ignore openings, but this is the exception rather than the rule.

Because it is just silly

There is a difference between foolish moves that have a logic behind them ( which your opponent may or may not get ) and foolish moves that have zero logic behind them. The latter ones I won't try.