The curious case of Magnus' 5.Bc4 in the Open Sicilian

The curious case of Magnus' 5.Bc4 in the Open Sicilian

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Last week, in the Magnus Invitational, Magnus Carlsen used the new move 5.Bc4 in the Open Sicilian first against MVL and a few rounds later against Nepomniachtchi.

Position after 5.Bc4
Was this a mouse slip or a deliberate new idea? From the fact that Carlsen repeated the move against Nepomniachtchi, we can conclude this was a deliberate choice.
The pawn on e4 is unprotected and can be taken. In addition, if Carlsen wants to avoid the Sicilian Najdorf, what will he play after 5... e6? After 6.Nc3, Black can play 6... a6 transposing to the Sozin-variation of the Najdorf and after 6... 0-0, Black can take on e4 and follow-up with d5 protecting the knight on e4. The mystery becomes even stranger as Carlsen was lost with White against Nepomniatchi after only 7 moves!

How is this possible? Carlsen commented after the game:

"That was pretty sick! I just completely blanked after e6. I knew that this was fine for White, and I’d gone over it before the game against Maxime, and then then I just completely blanked there. I couldn’t remember what to do! I knew these lines with Bb5+ deep into them, but for a second there I didn’t know how I should get there at all, and so I just sat thinking for 5-6 minutes and my mind was just blank – there was just nothing. I saw that 7. Nxe6 loses a piece, I just at the board didn’t even consider Bb5+, even though I’d reviewed it before and knew it was the correct move. That was just total insanity."

How is this possible that a world champion, who is only 29 years old, and is known for his photographic memory, just blanks out, forgets his preparation, and can not reconstruct it over the board knowing that Bb5+ is part of the solution. Curious, right? 

Carlsen, who was able to recall the areas, population numbers, flags and capitals of all the countries in the world by the age of five, won many chess history quizzes and most recently was able to identify 8 out of 9 game position from Anand (a quiz organized for Anand's 50th Birthday). How can you suddenly lose that capability at the age of 29?
Let's return to the chess variations. The move 5.Bc4 is covered in the April 2020 Hiarcs Opening Book, a book created based on engine, correspondence, and top GM games. Judging from the stated rating it has been played before between engines and the opening book suggests as the critical line (including 7.Bb5+):

A pretty crazy line and one can imagine that Carlsen wanted to use it as a surprise weapon.
Finally, let's return to the game against MVL who chose 5... e6. Magnus did not defend his pawn with 6.Nc3 but played 6. O-O, leaving the pawn on e4 again en prise. MVL did not test Carlsen's preparation and chose 6... Be7. What would have happened if Black had taken on e4? Stockfish 11 likes Black's position, Lc0 believes White has compensation.
5.Bc4!? leads to a very unbalanced position, where superior preparation can be a significant advantage, especially in rapid and blitz games. It was shocking to see the world champion completely blank out and forget his preparation as early as move 7.