Chess Opening Theory/1. Nf3/1...f5/2. e4

Lisitsin Gambit
a b c d e f g h 8 8 7 7 6 6 5 5 4 4 3 3 2 2 1 1 a b c d e f g h
Position in Forsyth-Edwards Notation (FEN) rnbqkbnr/ppppp1pp/8/5p2/4P3/5N2/PPPP1PPP/RNBQKB1R b
Moves: 1. Nf3 f5 2. e4

Lisitsin Gambit

2. e4 is a very counter-intuitive move, as after the pawn is captured White does not get to make a developing move but must move an already developed piece again. However, practice has shown that the knight landing on g5 is hard to chase away and creates certain tactical threats (more than simply Nxe4!) that compensate for the lost pawn. This gambit contains various traps that black must be careful not to fall into, but will lead to approximate equality if black knows what they're doing.

2...fxe4 is the only way to challenge White's idea. "The way to refute a gambit is to accept it." - Wilhelm Steinitz

2...e5 transposes to the notorious Latvian Gambit. This may actually be a good practical choice for someone who knows nothing about the Lisitsyn, as it gives Black a greater initiative, but if white knows the theory in this line as well, black doesn't get much out of it.