Chess Opening Theory/1. e4/1...e5/2. Nf3/2...d6/3. d4/3...Nd7/4. Bc4/4...Be7/5. dxe5/5...dxe5/6. Qd5

6.Qd5
This move threatens immediate mate and wins on the spot. The only way to prevent mate is Nh6, but after Bxh6, Black has to play O-O or Rf8 to stop mate and remain a piece down. If Bb4+, then after c3, the only way to stop the mate is Qe7 of Qf6 but they allow cxb4, winning a piece. So after Qd5, black must lose at least a piece.