I have noticed people finger tap on the table while tea or other drinks are being served. Today I decided to ask, and the story told is similar to this...
The Origin of Finger Tapping - Chinese Tea History
http://chinesefood.about.com/od/chinese ... apping.htm
The next time you're dining at a Chinese restaurant, try to discretely observe what happens at other tables when the tea is poured. You may spot someone tapping the table with three fingers each time their cup is refilled. Appearances to the contrary, this is not a superstitious gesture. In fact, the story behind finger tapping or tea tapping dates back to the Qing Dynasty (1644 - 1911 A.D.).
According to legend, one of the Emperors during this period was fond of traveling throughout the country in disguise, in order to observe his subjects unnoticed. At one teahouse he was particularly impressed by the way people were able to lean across the table and pour tea without spilling a drop. The Emperor decided to try pouring tea for his companions. Predictably, he ended up spilling it everywhere.
The Emperor decided he needed more practice. There was, however, one slight problem. Custom demanded that people bow before the Emperor. This, of course, would ruin his disguise. Instead, the Emperor told his companions to "bow" with their middle three fingers each time he refilled their cup - two fingers represented the prostrate arms and another the bowed head. Nowadays, tapping the table is a way of paying silent thanks to the person who poured your tea.