The fork trick is a tactical device with a positional goal, to depreciate an enemy pawn center.
Richard Reti White
Dunkelblum Black
Vienna 1914
Three Knights Game C46
1 e4 e5 2 ♘f3 ♘c6 3 ♘c3 ♗c5
Normal and good alternatives include 3...♘f6 and 3...g6.
4 ♘xe5! ♘xe5 5 d4
White regains his piece and ends up with a superior pawn center, 5...♗d6! 6 dxe5 ♗xe5, with a small plus.
5...♗xd4?!
Worse was 4...♗xf2+? 5 ♔xf2 ♘xe5 6 d4!. Then White’s king is quite safe, he has the two bishops and, most important, he has a far superior center. Note that Black can set a trap, 6...♕f6+ 7 ♔g1 ♘g4 (8 ♕xg4?? ♕xd4+).
Question 15: What’s the best reply?
6 ♕xd4 ♕f6?
7 ♘b5! ♔d8? 8 ♕c5!
Black resigned in view of 8...c6 9 ♕f8 mate. He could have tried 7...c6 based on 8 ♘c7+ ♔d8 9 ♘xa8?? ♘f3+ and 10...♕xd4. But White should win eventually after 9 ♗g5! ♕xg5 10 ♘xa8.
More miniatures with the fork trick:
Leonid Shamkovich White
Peter Dely Black
Moscow 1962
1 e4 e5 2 ♘f3 ♘c6 3 ♗b5 a6 4 ♗a4 ♘f6 5 0-0 ♗c5 6 ♘xe5! ♘xe4 7 ♕e2! ♘xe5 8 ♕xe4 ♕f6? 9 d4! ♗xd4 (Based on 10 ♕xd4?? ♘f3+) 10 c3! ♗c5 11 ♗f4 resigns.
Maxim Novikov White
Vladimir Afromeev Black
Tula 2007
1 e4 e5 2 ♘f3 ♘c6 3 ♗c4 ♘f6 4 ♘c3? ♘xe4! 5 ♗xf7+ ♔xf7 6 ♘xe4 d5 7 ♘eg5+ ♔g8 8 d4 h6 9 ♘h3 ♗g4 10 dxe5 ♘xe5 11 ♘f4 c6 12 h3 ♗xf3 13 gxf3 ♕f6 14 ♗e3 ♖e8 15 ♔f1 ♔h7 16 ♖g1 g6 17 c3 ♗d6 18 ♖g3 ♖hf8 19 ♔g2 ♘c4 20 ♖xg6 ♖g8 White resigns.
Alfonso Romero White
Fernando Braga Black
Leon 1990
1 e4 e5 2 ♘f3 ♘c6 3 ♗b5 ♘f6 4 0-0 ♗c5 5 ♘xe5 ♘xe5 6 d4 c6 7 f4 ♕b6 8 dxc5 ♕xc5+ 9 ♔h1 ♘eg4 10 e5 ♘f2+ 11 ♖xf2 ♕xf2 12 exf6 cxb5 13 ♘c3 d5 14 ♗e3 ♕h4 15 fxg7 ♖g8 16 ♘xd5 ♖xg7 17 ♘c7+ ♔f8 18 ♕d6+ ♔g8 19 ♗f2 resigns.