Black makes a temporary sacrifice of the knight, and if White plays 5.Nxe4, Black can play ...d5 with a fork which wins back the piece. This tactical trick does not win any material for Black, but knocking out the important White center pawn on e4 has secured Black a good foothold in the center.

Even so, 5.Nxe4, falling in with Black's plan is White's best move. 5.Bxf7+ looks good at first glance because Black's king gets slightly exposed, but White is not developed enough to sustain an attack. 5.O-O can cause problems for black if they are not careful.

The tactic employed by Black is called the "Fork Trick" and pops up in a number of openings, but it is particularly potent in the Four Knights.

For explanation of theory tables, see theory table and for notation, see algebraic notation..
1. e4 e5 2. Nf3 Nc6 3. Nc3 Nf6 4.Bc4 Nxe4!

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Wikipedia has related information at Four Knights Game

• Evans, Larry D. The Fork Trick. Published in two parts (April 2000 and May 2000) issues of Chess Life. Online facsimiles of the articles are available at chesscamp.net.