Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. Nc3/4...fxe4/5. Nxe4/5...d5/6. Nxe5/6...dxe4/7. Nxc6

Black's queen is attacked, and capturing the attacker with 7...bxc6 will end in tears after 8.Bxc6+ Bd7 9.Qh5+! So we have to move the queen. Moving the queen to the fourth rank seems like a good idea, to try and neutralise the discovered check by hitting the b5-bishop, and at the same time cut out any of White's Qh5+ ideas.

Sure enough, the two variations from this position are both queen moves to the fourth rank:
 * 7...Qg5 is the Classical Schliemann. The queen counter-attacks the helpless g2-pawn.
 * 7...Qd5 is the Möhring Variation. The queen threatens the bishop and, indirectly, the knight with capture. White's only move to hold things together will be 8.c4, after which Black can argue that White's d-pawn is backward on an open file. Whether this is actually relevant with so many tactical possibilities on the board is another matter.