The Empty Cup is an Old Zen anecdote. The Master, while serving Tea, Keeps Pouring, so that the Tea gets Wasted. The Idea is to show that We Cannot Receive Unless our Minds are Free to Receive, Un-Encumbered by our Past.
The Main point is that We have to Forget, even Give Up, What We have Learnt Earlier.
Let me talk of
THE EMPTY FLASK!
In Tamil cinema, I came across a comedy where a boy and girl work at the office; one wants tea and the other coffee, the office has one flask, and the office boy brings both the coffee and the tea in the same flask.
The Flask has to be Empty if You are going to Enjoy Your Drink.
The Mind has to be Emptied of all old teachings if it is to Grasp and Profit from New Teachings.
Christ, incidentally, had used the example of the Wine Skins, things of His time.
The Cup has to be Both Clean, and Empty. Half Empty will not do.
GAD Demands the Same. That one comes to learn Giving Up what One might have learnt Earlier.