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e f g 1 h Why it works
5...d5 looks completely natural: the e4 knight is pinned down the e-file, so it needs a pawn behind it — exactly how Black plays in the Open Ruy Lopez. The difference is that here Black has not gained the tempi with ...a6 and ...b5: the bishop still sits on b5, eyeing the c6 knight. White doesn't rush to grab the knight — 6.d3! simply chases it away, and after 6...Nf6 the e5 pawn hangs. 7.Nxe5! takes it while attacking the c6 knight, and 7...Nxe5 is impossible because of the pin on the e-file. Black tries to hold on with 7...Be6, but 8.Nxc6 bxc6 9.Bxc6+ comes with check and hits the a8 rook at the same time. Black's whole structure collapses in three moves.
Refutation
5...Nd6! — the main line of the Rio Gambit. The knight retreats to d6 and attacks the b5 bishop in return, releasing the pressure on the e-file and forcing White to commit. After 6.Nxe5 Nxe5 7.Rxe5+ Be7 the game follows well-trodden theory with rough equality, whereas 5...d5 simply drops material.