411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...d5/2. exd5/2...Qxd5/3. Nc3/3...Qa5/4. b4/4...Qxb4/5. Nb5/5...Qa5/6. Bc4
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Scandinavian Defense, Leonhardt Gambit
6. Bc4
It looks like White simply develops her bishop to c4. 6...c6 looks like an amazing move! You attack the annoying knight on b5 and give your queen an escape square in c7 just in case. Surely nothing can go wrong.
Right?
It turns out, 6. Bc4 has a threat of Bxf7!! followed by Qh5+ and then Nd6+ winning Black's Queen! That is why 6...Nf6 is black's best move here, you cover the h5 square and avoid white's tricks.