Chess Opening Theory/1. e4/1...e5/2. Qh5/2...Nc6/3. Bc4/3...g6/4. Qf3/4...f5

=Frederiksberg Variation=

4...f5
Black 'sacrifices' the f5 pawn, although it can't actually be taken, because of 5...Nd4. White's queen is stuck in the center and is a target for Black's knights, bishops and pawns. Black can sometimes play f4, with the idea of Bg4 trapping the queen.

White will have to play very carefully to not end up dead lost.

Theory table
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'''1. e4 e5 2. Qh5 Nc6 3. Bc4 g6 4. Qf3 f5'''