The story tells of a Qing Dynasty emperor who used to go out and about on his lands on incognito inspection visits. While visiting South China, he once went into a teahouse with his companions. In order to preserve his anonymity, he took a turn at pouring tea as not to have done so would have revealed his special status. His shocked companions wanted to kowtow to him for the great honor he was doing them. Instead of letting them reveal his identity, the emperor told them to tap three fingers on the table. One finger represented their bowed head and the other two represented their prostrate arms. Apocryphal or not, the relatively modern tale is the basis for the charming custom of discreetly tapping your acknowledgment of a tea-drinking companions consideration. 

There is another yum cha ritual that has an historic origin. When you see tea-drinkers tapping the table with three fingers of a hand, do not think it is a superstitious gesture. It is a silent expression of gratitude to the member of the party who has refilled their cup. The gesture recreates a tale of Imperial obeisance.