It's a question from a fan on brushing when serving and looping and how to train on it.
Guo says the key is thin contact. One could train off the table, by "slicing off" the bottom of the ball. Starting out, the blade angle could be more vertical, and then more horizontal as you learn to "grab" the ball. Another thing is that the blade angle does not remain static throughout the stroke, and the blade path is not strictly linear, but more of a curve. Rather than just the wrist, the arm and the body are the main contributor during the stroke.