Line paint may refer to placing a sheet of paper in the easel, after focusing, etc like normal, under safe lighting.
Then use developer (straight, and maybe warm) with a brush to paint areas of the image, gradually bringing up values. Combined with turning the enlarger on and off at times.
Go into normal diluted tray of developer or straight to the stop bath.
Much trial and error may yield interesting results.
Can be fun, especially for learning students. The younger the better.