Answer: Obviously, Moriarty has real mahjong (2 points for Dragon Pung and 6 points for Melded Hand) so Sherlock cannot pass discard of . Though, for winning table and championship he needs mahjong exactly for 8 points.

Sherlock declares "Hu", takes discard and start to count with discarded tiles including four own Flowers:

Totalling 2+2+1+1+1+1=8 points.

"But wait!" shouted Moran. "Why you did not score Pure Straight? And what did you do with Flowers?"

"Relax, he is right," said Moriarty. "He acts on the basis on "Green Book".

"Green Book"

"Green Book" has several rules on how the hand is scored. Sherlock simply used them for his advantage.

Firstly, hand is scored not by judge, not by table (by all four players like in riichi) but solely by player. "The player himself must declare (adding up the Fan Points of his hand), then the other players may check, and the referee verifies". This means the nobody can interfere player.

Secondly, whenever there is a choice then player can choose. In p. (4) it is said:

(4) Freedom to Choose the Highest Points ("the High-versus-Low Principle"):If you can use a set to form both a high-score fan and a low-score fan, it is your right to choose the high-score fan.

This time once in lifetime of all players winner have decided to choose low-scoring fans.

Finally, if tiles of a hand are used for counting this is a foul, though if Flower tiles are used then hand stays but player loses points for used to score flowers.

P.S. This mystery is based on real life play at Russian MCR Open 2015 (though player at Moriarty position declared "Hu" prior to Sherlock).