Hartlaub Gambit
English Defense
1.d4 b6 2.c4 Bb7 3.Nc3 e6 4.e4 f5 5.exf5 Nf6
White plays e4 anyway, and Black hits it with ...f5, giving a pawn for open lines.
Practice this lineThe idea
After 4.e4 f5 5.exf5 Nf6, White has won a pawn and Black has quick development and open lines. Lichess calls this the Hartlaub Gambit Accepted; White can also decline it with 5.d5.
Plans for White
Develop with Nf3, Bd3 and castling, and give back the pawn if needed to get the king safe. White can also try fxe6 to grab more material, but that opens lines toward the king.
Plans for Black
Develop quickly with ...Bb4, ...O-O and ...Nc6, and attack e4 and White's king. Taking back on f5 or e6 at the right moment regains the pawn. Black's lead in development is the compensation.
Watch out for
For White, the long diagonal. With the e-pawn gone, the bishop on b7 hits g2 and h1, and White's kingside can come under fire.
For Black, a slow attack. If White develops safely and keeps the pawn, Black is simply a pawn down.