Strange pawn double move in McDonnell attack I was going through a Chessable opening line, and it recommends black to play 6...d5, moving the d pawn immediately after it was just moved in 5...d6 6.Nc3.

What is the reason behind moving the same pawn twice?

[FEN "r1bqk1nr/pp2bppp/2npp3/2p5/4PP2/2NP1N2/PPP1B1PP/R1BQK2R b KQkq - 1 6"]

6... d5 7. O-O Nf6 8. e5 Nd7 9. d4 cxd4 10. Nb5 O-O 11. Nbxd4 Nxd4 12. Nxd4 Nc5


Full line here:

[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]
1. e4 c5 2. f4 Nc6 3. Nf3 e6 4. d3 Be7
5. Be2 d6 6. Nc3 d5?? 7. O-O Nf6 8. e5 Nd7
9. d4 cxd4 10. Nb5 O-O 11. Nbxd4 Nxd4 12. Nxd4 Nc5
I checked the position on Stockfish and its 1st or 2nd recommendation is to play 6...d5 after 6.Nc3, so your Chessable opening line is sound.

The reason for playing 6...d5 is that White's c3-knight is misplaced. If you were to attack it by pushing ...d4, it doesn't have a good square to move to (normally, it would like to move to e2 but a bishop is sitting there).

The reason for not playing 5...d5 on the fifth move is that White doesn't have to bring his knight to c3. Instead, he can move it to d2, where it won't be attacked by your pawn.

In the line you posted, I think White went for the stuff with 8.e5 and 9.d4 in order to prevent Black from playing ...d4 at an opportune moment. The pawn structure that arose after 9.d4 is favorable for Black, since White's missing his centre pawn.