Psychology Trivia : Tapping the table by the finger bit by bit

It is a signal of wanting you the early round-up of the talk that tapping the table by the finger bit by bit.

It is the result that the will of the interference is reflected on movement of the hand unconsciously. 

It has the same meaning as which beating by not the finger but the pen, etc. 

The desire of obstructing your talk by the sound of tap-tap works unconsciously. 

Moreover, it's also an expression of refusal to you to touch the coffee cup or ashtray on the table more than necessarily.