411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Nf6/4. Ng5/4...d5/5. exd5/5...Ne7
Reference material by Wikibooks contributors. This retains its source identity and is not 411 news reporting.
Two knights defence C57 Two knights defence → 4. Ng5 → 5. exd5
5...Ne7?
Black retreats their knight and brings another attacker to the d-pawn.
White is very happy to play 6. Nc3 to defend the pawn.
A sharper line is 6. d4. Were White to play 6. d6? to open up the attack on f7 again, Black would be able to play 6...Ned5, and the knight, supported by the other one, obstructs the bishop's attack. Including 6. d4 first however means that if 6...exd4? 7. d6 Nebd5, White has 8. Qe2+, preventing Black's king from defending f7. If 6...Nxd6? 7. dxe5! with tempo.