Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Nf6/4. Ng5/4...Bc5/5. Nxf7/5...Bxf2/6. Kxf2/6...Nxe4

6...Nxe4+
Black plans to bring their queen out with Qh4 next move. To avoid any danger and move out of the open file, White has two sensible options: Kg1 and Ke3.