The Chawan Sign is a code in which members of the same martial arts school use to communicate with each other with tea cups at places where people dine.
Guixiang Lee explains the Chawan Sign to Ryo Hazuki after he finds the Chawan Sign paper that was inside The Wulinshu book written by Yuanda Zhu that he found at the Man Mo Temple library. She tells him to make the sign on the back of the piece of paper, so that if an associate of Zhu sees it, they will make contact with him. After Ryo does this at diners, he will eventually come into contact with Yuanda Zhu's aide Shuqin Zhang, who after being captured by the Yellow Heads shortly after they meet and then rescued by Ryo, suggests he come into contact with Ren of Heavens.
Later on, in Kowloon when Yuanda Zhu is captured by the Yellow Heads and all the entrances to the Yellow Head Building where he is being held captive are heavily guarded and blocked off, Ren suggests he and Ryo split up to do the Chawan Sign at tea shops in different quarters for information. Ryo doing the Chawan Sign again leads him to Kai who will give him a new lead after they meet.