After translation of previous comic and before this one, Shizuki and Ran wanted to strip Tamaryu, so she challenged them to strip mahjong to make it sporting.

And since they needed a fourth player, they roped in Catherine, who is clearly still reading the manual, and who yet has apparently schooled all three of the others in a single blow with a hand that I don't know the name of, but I do know that it's extraordinarily difficult (one each of all three dragons, all four winds, and a one and nine of each suits).