Now for the 3rd out of 4 digressions into C-K theory - in part 4 we'll look at the modern 7....Nf6, but for now we'll stick with the main line and examine the consequences of an early ...Qa5+/...Bb4+, which have been tried as an improvement for Black.
Starting as always with the key position after 10. QxB:
11. Bf4 Bb4+!? is possible, on the premise that a c2-c3 advance is not in White's favor since it provides a lever for Black in a later pawn attack. And if White plays an eventual c3-c4 then the tempo that was 'sacrificed' is regained. And so,
12. c3 Be7
leaves White a choice - in the stem game Karpov-Larsen 1982
White chose the natural 13. Ne4 aimed at d6, but Larsen chose the surprising 13. .... Ngf6! when 14. Nd6+ BxN 15. BxB Qa5 left White with castling problems, though White did manage to win all the same.
More often is played 13. O-O-O when ....Ngf6 14. Kb1 a5 15. Ne5 leads to a slight edge for White - he does best to reserve c3-c4 for a later time.
More testing is:
11. Bf4 Qa5+
12. Bd2 Bb4 (12. c3 is not well regarded as the Black Queen is well posted on a5)
13. c3 Be7
14. c4 Qc7 (the persistent 14....Bb4 is not well regarded since after 15. Ne4 Black is under pressure to exchange into an unfavorable endgame)
15. O-O-O Ngf6
and now Black has induced c2-c3-c4 without apparent loss of tempo, he has the added move Qc7. However, this variation is strategically complex because the position of Black's queen is not always beneficial nor is the pawn on c4 always a burden. It sharpens the game, as Black's queenside breaks (c7-c5 or b7-b5, even a7-a5) get more bite, but White has more play in the center.
White has several options at move 16, I won't beat all of them to death, but here are a couple:
16. Ne4 O-O
17. NxN+ NxN
18. g4!? Nxg4 (note the thematic pawn sacrifice)
19. Rhg1 f5
with mixed results - this line is essentially identical to the early c4 lines in the last post where Black also has the sharp option of 16. .... b5!? when White often responds with 17. c5 and the game resembles Ivanchuk-Seirawan 1990 posted earlier.
In one notable game White even skipped 17. NxN+ and went for the immediate 17. g4?!?, and arrived in the following position in Bacrot-Leko 2008
White to move....why so serious?
16. Qe2 O-O (16. Qe2 is an exceptionally tricky move that looks to take advantage of the weakness of Black's e-file - think of g2-g4 followed by Nxg4, Rhg1 f5, Qxe6+; also, a knight sacrifice on f5 is threatened because the bishop on e7 is undefended)
17. Kb1 Rfe8
18. Ne5 c5!?
19. Bf4 With a tiny edge to white who has placed his pieces well but his king is more exposed
16. Kb1 O-O as in other variations a quick Kb1 has become the vogue at the top level. Why? Flexibility - White can play on either the d-file, c-file, or g-file, and waits for Black's response. However, this is not the most critical move.
17. Ne4 Rad8
18. NxN+ Nxf6
Another popular choice is 16. Rde1/Rhe1, putting pressure on e6/e7. Lots of choices! The theoretical debate is still going as can be seen from the recent games.
However, recently there's also been fights around a line that tries to underline a unique weakness in Black's approach - after
12. Bd2 Bb4
13. Ne4!? Pointing to the hole on d6 and giving Black the choice of either a slightly inferior endgame after exchanges on d2 or....
13. .... Ngf6!
14. Nd6+ Ke7
and it transpires that Black can get away with this audacious play because White will have difficulty mounting a strong attack against the Black king. White can 'win' a pawn with
15. Nxb7 Bxd2+
16. Nxd2 Qb4!
17. Qa3 QxQ
18. PxQ but it is difficult for White to make something of the extra pawn (although for Black to win is nearly impossible)
Alternatively the main line has become:
15. Nc4 Bxd2+
16. Nfxd2 Qc7
17. O-O-O Rhd8
where White has a slight edge because of his space advantage.
Conclusion: 11. .... Qa5+ is a topical line that is growing in theory, and is an interesting alternative for Black. However, White has many options to choose from in fighting it, and seems to be able to draw at will if he is so inclined. Meanwhile the sharp c4 lines are still scoring wins for White.