en passant: A pawn which makes the two square initial move may be captured by an enemy pawn (not a piece) as if it had only moved one square provided the capture is made on the next move.

1...g5

Not aware of the rule of en passant and hoping to promote to queen at the earliest.

2.hxg6

White well aware of the en passant rule converted the lost position to a winning position. When his opponent disputed this rule, he asked his opponent to visit http://www.chesskit.com and learn all about en passant.

[2.Kb2

now that White has not opted to take the en passant at g6, White loses the right to choose en passant here after.

2...g4

Black simply wins as the white King can not capture the Black pawn before it promotes on g1 as the White king is outside the cardon square. White just lost a winning opportunity as he was not aware of en passant either.]