Spanish Countergambit, Harding Countergambit, Fricke Gambit

The Fricke Gambit is White's 5. O-O against the 4...Qg5 line of the Spanish Countergambit. Instead of retreating with 5. Nf3 or taking on c6 with the Harding Gambit, White castles. This takes the sting out of ...Qxg2, since the king now guards that pawn. The knight on e5 stays where Black's queen attacks it, so White, who is a pawn up, offers material in return for a lead in development and a quick castled position.