Chess Opening Theory/1. e4/1...e5/2. f4/2...exf4/3. Nf3/3...d6/4. d4

d4 grabs the d4-e4 classical center and opens up the c1 bishop's attack on the f4 pawn.

Black usually responds with g5, temporarily defending the pawn while planning ...g4 to attack the f3 knight and expose White to Qh4+.

The piece sacrifice line: Nc3 g4!? Bxf4 gxf3 Qxf3 Nc6 Bb5 Bd7 O-O-O Qh4 Kb1! (get out of the way of diagonal) O-O-O. This is unclear.

Some play h4 to attack the g5 pawn and prevent Qh4+. This is around equal with best play.