Chess Opening Theory/1. e4/1...e5/2. f4/2...exf4/3. Nf3/3...g5/4. h4/4...g4/5. Ng5

=Allgaier Gambit= At first it looks like the knight trapped itself on g5, since it has no safe retreat squares. But the idea behind 5.Ng5 is to sacrifice the knight on f7 and force the black king out into the open. This gambit has a better chance of succeeding in a blitz game where tactical errors are more frequent in sharp games like this, rather than in a correspondence game against a prepared opponent where there might not be enough compensation for the lost piece.

Black can accept the challenge with 5...h6 forcing the knight sacrifice; or he can steer the game in a more peaceful direction (and avoid the sacrifice) with the solid equalizing move 5...d5.

Theory table
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1.e4 e5 2.f4 exf4 3.Nf3 g5 4. h4 g4 5. Ng5