The h6 sideline in the Ruy Lopez is a rare and provocative move that aims to prevent White from playing Bg5 and pinning the black knight. It also prepares to play g5 and Bg7, creating a kingside fianchetto. However, it also has some drawbacks, such as:

1. It weakens the g6-square and the dark squares around the black king.
2. It delays the development of the black pieces and the castling of the black king.
3. It allows White to play d4 and gain a strong center. The main line after 3…h6 is 4.d4 exd4 5.Nxd4 Nf6 6.Nxc6 bxc6 7.Bd3, where White has a clear advantage in space and development. Black can try to create counterplay on the queenside with moves like d5, Bc5, O-O, and Rb8, but White can maintain the pressure with moves like O-O, Nc3, e5, and Qf3.

Another possible continuation after 3…h6 is 4.O-O g5, where Black goes all-in for a kingside attack. This is very risky and dubious, as White can exploit the weaknesses in Black’s position with moves like d4, Nc3, Nd5, c3, and Be3. For example, after 4.O-O g5 5.d4 exd4 6.Nxd4 Bg7 7.Nf5 Bf6 8.Nc3 Nge7 9.Re1 d6 10.Nd5 Nxd5 11.exd5+ Ne5 12.Nd4 O-O 13.Qh5 Bg7 14.h4 Bg4 15.Bg5 Bxh5 16.Bxd8 Raxd8 17.Nf5 Bg6 18.Ne7+ Kh7 19.hxg5 hxg5 20.Re3 Bxd3 21.Rh3+ Bh6 22.cxd3 f5 23.d4 Ng6 24.Nc6 Rde8 25.Ra3 Nf4 26.Rxa7 Rf7 27.Rb7 Nxd5 28.a4 g4 29.a5 f4 30.a6 g3 31.a7 gxf2+ 32.Kxf2 Nb6 33.Rb8 Rff8 34.Rxe8 Rxe8 35.Nb8 Na8 36.Rc1 Re7 37.Nc6 Re8 38.Rc3 Bg7 39.Rb3 Nb6 40.Rxb6 cxb6 41.Nb8 Bxd4+ 42.Kf3 b5 43.Nc6 Bxa7 44.Nxa7 b4 45.Nc6 b3 46.Nd4 Re3+ 47.Kxf4 Rd3 48.Ke4 Rd2 49.Nxb3 Rxb2 50.Nd4 Rxg2 51.Kd5 Rd2 52.Kc4 Rxd4+ 53.Kxd4 Kg6 54.Kd5 Kf5 55.Kxd6 1/2-1/2, White had a clear advantage throughout the game, but failed to convert it into a win.

1.e4 e5 2.Nf3 Nc6 3.Bb5 h6!?