411 REFERENCE · Wikibooks
Chess Opening Theory/1. f4/1...e5/2. fxe5/2...d6/3. Nf3/3...dxe5/4. Nxe5/4...Bd6
Reference material by Wikibooks contributors. This retains its source identity and is not 411 news reporting.
From's Gambit rd nd bd qd kd nd rd pd pd pd pd pd pd bd nl pl pl pl pl pl pl pl rl nl bl ql kl bl rl Bird's Opening
4...Bd6
This attacks the knight. Since 5. d4? Bxe5 6. dxe5 Qxd1+ is uncomfortable for White, taking on double isolated e-pawns and forfeiting castling rights, while 5. Nc4?? Qh4+! 6. g3 Qxg3+!! 7. hxg3 Bxg3# is lost, and 5. Nd3? Qh4+ 6. Nf2 Bxh2 is equal, White is well advised to transpose to mainline theory with /5. Nf3/.
From's Gambit