The artist in the video is doing a Chinese art form called Inner painting or inside painted.
This art form involves glass bottles which have pictures and often calligraphy painted on the inside surface of the glass.
The bottles are produced by manipulating a specialized paint brush through the neck of the bottle.
To paint the inside of the bottle, the artist must paint backwards.
Concentration is crucial to make precise strokes.
A skilled artist may complete a simple bottle in a week while something special may take a month or more.
The best craftsmen may only be able to produce 85 bottles in a year.
This reminds me of the older vintage transistor radios with their reverse painted cases.