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

White ties the minor piece count at 3 with 7. Nxc6. It's a free knight because if Black recaptures with the b-pawn, Bxc6+ followed by Qh5+ gives Black big problems.